PATENT ABSTRACT
A machine gun, representatively a 30 mm machine gun, is mounted on an outer end portion of a support plank structure projecting out of the cabin area of a helicopter using a roller cradle assembly secured to the outer plank end. A variety of structural improvements are incorporated into the machine gun to (1) reduce its mechanical complexity, (2) reduce its firing recoil to make the gun more suitable for light aircraft mounting, and to improve the gun&#39;s firing controllability and accuracy, and (3) to make the gun easily and quickly field strippable, for cleaning, inspection and repair purposes, without requiring the services of a highly skilled armament mechanic or taking the gun to a specialized repair facility.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 09/007,090 filed on Jan. 14, 1998, now U.S. Pat. No. 6,176,169, which was a continuation-in-part of U.S. application Ser. No. 08/812,756 filed on Mar. 6, 1997, now U.S. Pat. No. 5,767,436. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to aircraft armament apparatus and, in a preferred embodiment thereof, more particularly relates to the mounting of machine guns, representatively 30 mm machine guns, on light aircraft such as helicopters. 
     The external mounting on aircraft of weaponry such as machine guns has heretofore carried with it a variety of structural, operational and safety limitations and disadvantages. To a great extent these problems have been eliminated by using a honeycombed metal support plank such as that illustrated and described in U.S. Pat. No. 5,419,234 to Sanderson. As illustrated in such patent, 40 mm machine guns are mounted on opposite support plank ends which project outwardly from the cabin area of a helicopter. 
     A particularly difficult external aircraft mounting problem is presented by the much higher recoil 30 mm machine gun. In previously proposed nonplank-based external aircraft mounting systems for 30 mm machine guns, several problems, limitations and disadvantages have been present. For example, due to the high recoil forces generated by firing the 30 mm machine gun, their external aircraft mounts have tended to be quite complex—an undesirable characteristic arising from the need to protect the “light” helicopter (such as an MD 500 or Bell 206 helicopter) from structural damage from recoil forces, and to prevent loss of pilot control of the aircraft during gun firing. Attempts to design an external aircraft mount structure for the 30 mm machine structure have heretofore not been entirely successful in either of these areas. 
     Other problems, limitations and disadvantages presented in the use of both aircraft and ground-mounted 30 mm machine guns have to do with the construction and operation of the gun itself. The three primary areas of such problems, limitations and disadvantages in conventionally constructed 30 mm machine guns arise from (1) their complex construction, (2) their heretofore unavoidable high firing recoil forces, and (3) the difficulty in field servicing the guns. 
     The complex construction of traditionally configured 30 mm machine guns can lead to reliability problems and difficult and expensive fabrication operations. The high recoil forces of conventional 30 mm machine guns undesirably diminishes their firing controllability and accuracy and, as mentioned above, has made it quite difficult to mount 30 mm machine guns on light aircraft such as helicopters. Further, the difficulty in field servicing the guns typically requires that they be uncoupled from the aircraft and taken to a special maintenance facility to be worked on by a specialized technician having an aircraft mechanic/ordnance skill level. This has been true even for routine gun cleaning and maintenance services. Additionally, to simply clean a conventionally constructed 30 mm machine gun it has to be taken nearly completely apart—a tedious task entailing removing countless screws, bolts and other fasteners and then replacing them after the gun cleaning task is completed. 
     From the foregoing it can readily be seen that a need exists for a 30 mm machine gun that eliminates or at least substantially reduces the above-mentioned problems, limitations and disadvantages commonly associated with 30 mm machine guns of conventional construction. It is to this need that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a machine gun, representatively a 30 mm machine gun, is provided with a variety of unique structural and operational features which serve to (1) reduce the complexity of the gun, (2) make it relatively easy to service in the field, and (3) substantially reduce its recoil to thereby facilitate its mountability on light aircraft such as helicopters, and improve its firing controllability and accuracy. The gun is illustratively secured, via a rollered mounting cradle, to the outer end of a support plank structure extending transversely through the cabin area of an aircraft, but may be supported in other manners in a variety of other aircraft and ground-based mounting applications. 
     In its preferred embodiment, the machine gun basically comprises a body upon which a barrel is mounted, the barrel longitudinally extending forwardly and rearwardly along a firing axis. The body is supported by a cradle and feed structure unit for rearward recoil movement and forward counter-recoil movement relative to the cradle and feed structure unit in response to the firing of the gun. The feed structure part of the unit is operative to position successive cartridges for retrieval and chambering in the barrel for firing therein. An operating rod structure is carried by the body for driven movement relative thereto cyclically in forward and rearward directions in response to firing of the gun, and a resilient recoil system is provided for absorbing the rearward recoil and forward counter-recoil forces of the gun. Firing apparatus is provided for firing each chambered cartridge, the firing of each cartridge causing a rearward recoil movement and subsequent forward counter-recoil movement of the gun body relative to the stationary feed and cradle apparatus. 
     Ammunition handling means are carried by the gun body and are operative to deliver cartridges from the feed structure to the barrel for firing therein, and then extracting and ejecting the spent casings of the fired cartridges. Representatively, the ammunition handling means include a bolt member carried by the operating rod structure rearwardly of the feed structure for cyclical forward and rearward movement with the operating rod structure toward and away from the feed structure, the bolt member having first and second extractor means thereon for releasably receiving and retaining rear cartridge casing rim portions. 
     A bolt face member is carried by the bolt member for movement relative thereto in first and second opposite directions transverse to the barrel, the bolt face member having ejector means thereon for releasably circumscribing a rear cartridge casing end portion. Cooperating means are provided on the bolt face member and the gun body for moving the bolt face member in the first direction relative to the bolt member in response to movement of the bolt member toward the feed structure, and for moving the bolt face member in the second direction relative to the bolt member in response to movement of the bolt member away from the feed structure. Representatively, these cooperating means include a cam track formed in the body with a pivotally spring-loaded switch plate member attached, and a roller structure carried on the bolt face member and received in the cam track for guided rolling movement therealong. 
     The relatively simple bolt member/bolt face member structure performs several ammunition handling functions during the firing of the gun—namely, (1) extracting a first cartridge from the feed structure, (2) chambering the extracted first cartridge in the barrel, (3) extracting a second cartridge from the feed structure while extracting the spent casing of the first cartridge, after firing thereof, from the barrel, and (4) moving the extracted second cartridge into a chambering alignment position in a manner causing the extracted second cartridge to engage and forcibly eject the spent casing from the ammunition handling means by ejectors mounted on the bolt face member. 
     Cooperatively engaged first and second structures are respectively disposed on the gun body and the feed structure for operating the feed structure in response to forward and rearward movement of the gun body relative to the stationary feed structure. Because the movement of the gun body relative to the feed apparatus operates the feed apparatus, no complicated timing system is needed. 
     Representatively, a cam track slot having an angled central portion is formed in a top side portion of the gun body and slidingly receives a cam follower pin which depends from a rotatable feed drum portion of the feed structure. The feed drum is connected by a one way clutched gear train to a splined drive shaft which extends parallel to the barrel and is rotationally locked to a sprocket structure that delivers individual cartridges from a cartridge belt to the bolt structure. The sprocket structure is slidable along the splined drive shaft, between front and rear limit positions and is spring-biased toward its rear limit position. A depressible bolt face sensor switch is carried on a rear side portion of the sprocket and is forwardly struck and depressed by the bolt face structure during firing of the gun. 
     During the firing of the gun, the rearward recoil of the gun body relative to the feed structure causes the cam pin/cam track slot interaction to rotate the feed drum in a first direction which back-indexes the ratcheted gear train without rotating the splined drive shaft, and the forward counter-recoil movement of the gun causes the cam pin/cam track slot interaction to rotate the feed drum in a second direction which rotationally drives the gear train to rotationally drive the sprocket structure and advance the ammunition belt one cartridge. 
     The operating rod structure of the gun preferably comprises a spaced pair of hollow left and right operating rods extending parallel to the gun barrel. The operating rods have closed front ends, and open rear ends anchored to an operating rod body slidingly carried within the gun body for forward and rearward movement relative thereto. The previously mentioned bolt structure is carried by the operating rod body for forward and rearward movement therewith within the gun body. Guide rods extend forwardly through the operating rods, and springs carried by the guide rods resiliently bias the operating rod structure to a front limit position within the gun body. When the gun is fired, pressurized gas from the fired cartridge is used to rearwardly drive the operating rod structure, against the resilient resistance of the guide rod springs which function to return the operating rod structure, and the bolt structure which it carries, to their forward limit positions. 
     According to a feature of the invention, the bolt structure is cyclically movable in forward and rearward directions within the gun body between a front limit position disposed rearwardly of the front limit position of the operating structure, and a rear limit position. A lock structure is associated with the operating rod structure and the bolt structure and is shiftable relative thereto between (1) a first position in which the lock structure releasably latches the operating rod structure and the bolt structure for conjoint forward and rearward movement relative to the gun body, and (2) a second position in which the lock structure releasably interlocks with the gun body in a manner preventing forward and rearward movement of the bolt structure relative to the gun body, and unlatches the operating rod structure and the bolt structure to permit forward movement of the rod operating structure relative to the gun body and bolt structure. 
     First means are provided for shifting the lock structure from its first position to its second position in response to the bolt structure reaching its front limit position, and second means are provided for shifting the lock structure from its second position to its first position in response to rearward movement of the operating structure away from its front limit position. 
     When the bolt structure with the bolt face structure attached thereto reaches its forward limit position, and is locked therein by the shifted lock member, the bolt face structure depresses the aforementioned bolt sensor switch. When the forwardly moving operating rod structure finishes the lock movement and the lock structure reaches its front limit position it causes a second switch to close. Only when both of these two switches close, assuring that the bolt is locked to the gun body and the gun is within the feed system, can the gun be fired. 
     In response to the operating rod structure reaching its front limit position, with the bolt structure locked to the gun body by the shifted lock member, a secondary mass member slidably carried behind the operating rod body portion forwardly strikes the operating rod body in a manner inhibiting undesirable rearwardly directed impact bounce-back movement of the operating rod structure which might unlock the bolt structure from the gun body. The secondary mass member is spring-biased rearwardly away from the operating rod body portion until the operating rod structure stops upon reaching its front limit position. 
     As the operating rods are cycled back and forth during firing of the gun, an electrical brush member slides along an electrically insulative strip on the outer side surface of one of the operating rods, the strip having an electrically conductive hot shoe portion at one end thereof and electrically coupled to the operating rod body portion via a wire imbedded in the insulative material. Shortly before the operating rod body reaches its front limit position the brush slides onto the hot shoe as the operating rod body closes the second firing switch. An electrical switch circuit receives a signal from the bolt face sensor switch when it is closed by the bolt face structure and responsively transmits electrical current to the brush to permit the gun to be fired when both switches close. In this manner, electrical firing power is sent to the gun only when needed—it need not be maintained continuously as in most conventional machine gun electrical firing systems. 
     The recoil system resiliently absorbs the recoil and counter-recoil forces of the gun as it moves rearwardly and forwardly relative to the stationary cradle mounting structure which supports the gun and holds the feed structure. In its preferred embodiment, the recoil system includes a recoil member fixedly securable to the stationary mounting structure, the gun body being movable forwardly and rearwardly relative to the recoil member, the recoil member having opposite front and rear portions. The rear portion of the recoil member is preferable formed from a resilient material. 
     The recoil system also representatively includes a counter recoil shock absorber anchored to the gun body and having a forwardly projecting depressible plunger member engageable with the rear portion of the recoil member; a recoil spring stop member anchored to the gun body forwardly of the recoil shock absorber, with the recoil member being disposed between the recoil shock absorber and the recoil spring stop member; a rod member having a first end anchored to the recoil member, and a second end slidably extending forwardly through the recoil spring stop member; a recoil damper anchored to the gun body forwardly of the recoil spring stop member and having a rearwardly extending depressible plunger member secured to the second end of the rod member; and a spring structure carried by the rod member and being compressible between the recoil member and the recoil spring stop member in response to rearward movement of the gun body relative to the recoil member. 
     In a pre-firing condition of the gun the operating rod structure is held in a rearward position thereof, against the resilient force of the guide rod springs, by a specially designed sear system. The sear system utilizes pivotally mounted sear members having roller portions thereon. The sear members are pivotally biased toward first positions in which they engage inner portions of arcuate, forwardly facing ledges formed on the closed front ends of the two operating rods. In such first positions the rollers are below “jam” angles of the ledge/roller interface areas and prevent the front ends of the operating rods from moving forwardly past the rollers. 
     To unlatch the sear system from the operating rods, and permit the operating rod structure to be spring-driven forwardly to initiate firing of the gun, a sear solenoid is electrically energized to forcibly move a core portion thereof. The solenoid core portion is coupled to the pivotable sear members through a mechanically advantaged linkage system which is driven by the core portion to pivot the sear members to second positions just above the pinch angle of the operating rod ledges, but not out of contact with the ledges. The arcuate ledges then rotate the sear member rollers out of engagement therewith to allow the operating structure to be forwardly driven to initiate firing of the gun. During firing of the gun, each time the ledges rearwardly pass the sear member rollers they permit the rollers to be moved inwardly to their second positions and then kick the rollers back outwardly from the ledges as the ledges forwardly pass the rollers. 
     During certain pre-firing conditions of the gun, such as when it is being reloaded, the operating rods are stationary in their forwardmost positions within the gun body. To move them back to their “on sear” ready-to-fire positions a specially designed charger system is provided. 
     Unlike conventional charger systems, the charger system of the present invention (which preferably carries the previously described sear system) is carried on the gun body for recoil and counter-recoil movement therewith. Accordingly, the charger system adds to the overall recoiling and counter-recoiling mass to advantageously lessen the recoil and counter-recoil forces. 
     Additionally, in contrast to conventional charger systems, the charger system of the present invention does not have a member which must engage and rearwardly move the operating structure to its “on sear” position and then be forwardly returned to its starting system to get it out of the way of the operating structure before the gun can be fired. Instead, the charger system of the present invention, in a preferred embodiment thereof utilizes a motor-driven pinion gear which is shiftable transversely to one of the operating rods into and out of driving engagement with a gear rack portion formed on a flattened exterior side surface portion thereof. 
     The pinion gear is rotationally driven, via an intermediate gear train, by an electric charger motor and a portion of the pinion gear forms the shiftable core portion of an electrical solenoid. When the charger motor and solenoid are energized with the operating rods in their forwardmost positions within the gun body, the pinion gear is first shifted into driving engagement with the gear rack and then rotationally driven to rearwardly move the operating structure to its “on sear” position at which time it is latched in such position by the sear system. 
     During rearward charging movement of the operating rod structure by the charger system, an electrically charged brush slides along a strip of electrically insulative material on the other operating rod. When the operating rod structure reaches its “on sear” position, the brush moves off the front end of the strip and is grounded to its associated operating rod. This grounding responsively de-energizes the charger motor to thereby stop the rearward motion of the operating structure and shift the pinion gear out of driving engagement with its associated operating rod rack. 
     In addition to the recoil reduction achieved by mounting the charger/sear system on the gun body for recoil and counter-recoil movement therewith, various other unique recoil reduction features are incorporated into the machine gun of the present invention. 
     For example, when the gun is fired the gun body rearwardly recoils. Subsequently, the operating rod structure is gas-driven rearwardly relative to the rearwardly moving gun body at a velocity greater than the velocity of the rearwardly recoiling gun body. Prior to the gun body beginning its forward counter-recoil stroke, the bolt structure carried on the rearwardly moving operating rod body portion strikes a resilient bolt buffer assembly carried on a rear interior end portion of the gun body. 
     This causes the operating structure and bolt structure to forwardly rebound in a manner such that the operating and bolt structures reach their front limit positions, and fire the gun, while the body is moving forwardly through its counter-recoil stroke. Thus, the firing recoil force is offset to a substantial degree by the mass of the still forwardly moving gun. 
     Another feature that desirably reduces the recoil force of the gun is a specially designed muzzle brake removably secured to the front end of the gun barrel. The muzzle brake has a hollow body coaxial with the firing axis of the barrel, and open front and rear ends. An axially spaced series of gas discharge openings are formed in the side wall of the muzzle brake body between its inner and outer surfaces, with the flow areas of the gas discharge openings progressively increasing in a rear-to-front direction along the length of the muzzle brake body. 
     The gas discharge openings are sloped rearwardly and laterally outwardly relative to the axis of the muzzle brake body. When the gun is fired, pressurized gas behind the cartridge projectile being expelled from the barrel is vented outwardly through the gas discharge openings, thereby exerting a forward force on the gun which desirably counteracts its rearwardly directed firing recoil force. The unique progressive cross-sectional increase in a forward direction of the gas discharge openings desirably tends to even out this forward force on the gun during pressurized gas expulsion from the muzzle brake gas discharge openings. 
     Another desirable attribute of the improved gas-operated, electrically fired machine gun of the present invention is that is easily field strippable for inspection, cleaning and maintenance purposes. For example, the gun is provided with a specially designed barrel lock system that permits the barrel to be rapidly installed and removed. To install the barrel, a rear end portion thereof in rearwardly inserted into a front end opening of the gun body. 
     The inserted barrel is then rotated ninety degrees about its axis to interlock lugs on the outer side of the barrel with corresponding lugs within the interior of the breech of the gun body to prevent axial removal of the inserted barrel. To releasably prevent unlocking rotation of the barrel, the inner end of a pin member transverse to the barrel is radially moved into an exterior side surface recess in the barrel, and the pin member is locked into place by the engagement of a rotatable pin handle with retaining pins in the gun body front end structure. 
     To subsequently remove the barrel, the pin handle is rotated to free it from the front end retaining pins. The barrel locking pin is then radially withdrawn from the barrel recess, the barrel is rotated to disengage its lugs from the gun body breech lugs, and the barrel is pulled out of the gun body. 
     According to another serviceability feature of the gun, a rear end cap portion of the hollow gun body is removably secured to the balance of the gun body, representatively by a pair of quick-release ball pins. The previously mentioned resilient bolt buffer structure and rear ends of the guide rods are secured to this removable end portion of the gun body. When the rear gun body end cap portion is removed, the bolt buffer structure and the guide rods and their associated springs come with it. Subsequently, and without using any tools, the overall operating rod structure, and the bolt structure which it carries, can simply be pulled rearwardly out of the gun. The gun body preferably has an open bottom side which further facilitates this easy and rapid field strippability of the gun. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2  and  3 , respectively, are phantomed side elevational, top plan and front end views of a representative light helicopter to which is transversely secured a cabin area support plank structure with specially designed 30 mm machine guns embodying principles of the present invention and carried on cradle structures opposite ends of the support plank structure; 
     FIG. 4 is an enlarged scale perspective view of the plank-supported aircraft armament apparatus shown in FIGS. 1-3; 
     FIG. 4A is a slightly reduced scale exploded perspective view of the FIG. 4 armament apparatus; 
     FIG. 5 is an enlarged scale perspective view of a machine gun portion of the armament apparatus; 
     FIG. 6 is an enlarged scale aft end perspective view of a support cradle portion of the armament apparatus; 
     FIG. 7 is a partially exploded perspective view of an aft end portion of the support cradle; 
     FIG. 8 is an enlarged scale perspective view of an eccentric cam roller assembly used in the support cradle; 
     FIG. 9 is an exploded perspective view of the cam roller assembly; 
     FIG. 10 is an enlarged scale aft end perspective view of the support cradle with ammunition feed structure portion of the armament apparatus secured to the top side of the cradle; 
     FIG. 11 is an enlarged scale aft end perspective view of a mount structure portion of the armament apparatus; 
     FIG. 11A is an aft end perspective view of an alternate embodiment of the mount structure; 
     FIG. 12 is an enlarged scale forward end perspective view of the assembled cradle, with mount portions of the armament apparatus, less the feed structure; 
     FIG. 13 is an enlarged scale, partially phantomed simplified cross-sectional view through the cradle structure, and the machine gun operatively supported therein, taken generally along line  13 — 13  of FIG. 6; 
     FIG. 14 is an enlarged scale aft side perspective view of a bore sight adjustment subassembly attached to the mount portion; 
     FIG. 14A is an exploded perspective view of the bore sight adjustment subassembly; 
     FIG. 15 is a laterally cut away left front perspective view of one of the 30 mm machine guns with its components in their open bolt, ready-to-fire orientations; 
     FIG. 16 is a laterally cut away left rear perspective view of the gun with its components in their open bolt, ready-to-fire orientations; 
     FIG. 17 is a laterally cut away left front perspective view of the gun showing its bolt moving forward from its “on sear” position in response to the initiation of the first shot from the gun&#39;s previous open bolt, ready-to-fire position; 
     FIG. 18 is an enlarged scale perspective detail view of the bolt, lock and feed system portions of the gun in their FIG. 17 positions; 
     FIG. 19 is an enlarged scale perspective detail of a sear assembly portion of the gun in its FIG. 17 position; 
     FIG. 20 is a laterally cut away left rear perspective view of the gun as shown in FIG. 17; 
     FIG. 21 is a laterally cut away left front perspective view of the gun, during chambering of an initial cartridge, with the gun&#39;s bolt face moving up into an upper cam path portion in preparation for capturing a new ready cartridge; 
     FIG. 22 is an enlarged scale perspective detail view of the bolt and feed portions of the gun in their FIG. 21 orientations; 
     FIG. 23 is a laterally cut away left front perspective view of the gun with the bolt unit capturing the ready cartridge; 
     FIG. 24 is an enlarged scale perspective detail view of the bolt and feed portions of the gun in their FIG. 23 orientations; 
     FIG. 25 is rear perspective view of the bolt and feed portions of the gun shown in FIG. 24; 
     FIG. 26 is a rear perspective view similar to that in FIG. 25 but showing the bolt unit “locking up” into the breech of the gun; 
     FIG. 27 is a rear perspective view similar to that in FIG. 26 but showing the bolt unit fully locked to the breech to permit an operating rod structure to fire the gun; 
     FIG. 28 is a laterally cut away left front perspective view of the gun illustrating its components in their orientations just after the gun is fired and is in its first, “free recoil” movement relative to its mount; 
     FIG. 29 is an enlarged scale perspective detail view of the bolt, lock and feed portions of the gun in their FIG. 28 orientations; 
     FIG. 30 is an enlarged scale laterally cut away left rear perspective detail view of a front end portion of the gun just after the initial firing thereof; 
     FIG. 31 is an enlarged scale left side perspective detail view of the gun&#39;s bolt, lock and feed portions during the start of the unlocking of the bolt from the breech subsequent to the firing of the first cartridge; 
     FIG. 32 is an enlarged scale right side perspective detail view of a gas pressure-driven operating rod portion of the gun initiating the unlocking of the bolt from the breech; 
     FIG. 33 is a laterally cut away left front perspective view of the gun, subsequent to the firing of the first cartridge, with its bolt unit fully unlocked from the breech and the gun and bolt in recoil travel; 
     FIG. 34 is a laterally cut away left front perspective view of the gun with its bolt face being cammed down and a feed drive gear being back-indexed for the next feed cycle; 
     FIG. 35 is a laterally cut away left front perspective view of the gun with the bolt having reached the limit of its recoil travel; 
     FIG. 36 is a laterally cut away left front perspective view of the gun in which it has reached its recoil travel limit; 
     FIG. 36A is an enlarged scale perspective detail view of a right side breech cutout area shown in FIG. 36; 
     FIG. 37 is a laterally cut away left front perspective view of the gun in which the bolt and gun are now both in full counter-recoil travel with the next cartridge feed cycle having been initiated; 
     FIG. 38 is a laterally cut away left front perspective view of the gun as the feed cycle continues with the bolt face moving upwardly; 
     FIG. 39 is a laterally cut away left front perspective view of the gun with the bolt now locked, the feed cycle continuing during forward movement of the gun, and the second cartridge in the chamber ready to be fired; 
     FIG. 40 is an enlarged scale right rear side perspective detail view of a portion of the gun entering and being aligned with the feeder during forward movement of the gun as shown in FIG. 39; 
     FIG. 41 is a partially cut away right rear perspective view of the gun feeder and cradle illustrating link ejection from the feeder; 
     FIG. 42 is a laterally cut away left front perspective view of the gun with its feed cycle completed and its bolt unit ready to capture a new cartridge; 
     FIG. 43 is an enlarged scale perspective detail view of the bolt and feeder portions of the gun in their FIG. 42 orientations; 
     FIG. 44 is a laterally cut away left front perspective view of the gun at the firing, under the automatic mode of the gun, of the chambered second cartridge; 
     FIG. 45 is an enlarged scale exploded perspective view of the bolt unit; 
     FIG. 46 is an enlarged scale perspective detail of a sprocket portion of the feed system illustrating its spring-loaded, splined connection to a drive shaft; 
     FIG. 47 is an enlarged scale perspective view of a right side portion of a spring and damper recoil system of the gun, with the system being in an at-rest state; 
     FIG. 48 is an exploded perspective view of a rear section of the recoil system portion shown in FIG. 47; 
     FIG. 49 is a view similar to that in FIG. 47 but with the recoil system portion being shown in its orientation created during firing recoil of the gun; 
     FIGS. 50-52 are enlarged scale cut away perspective views of the gun&#39;s sear assembly and sequentially illustrate its operation; 
     FIG. 53 is a partially cut away bottom rear side perspective view of the gun&#39;s charger/sear assembly illustrating portions of assembled sear system; 
     FIG. 54 is an enlarged scale perspective detail view of a drive ring portion of the sear assembly; 
     FIG. 55 is a partially exploded bottom front side perspective view of the charger/sear assembly; 
     FIG. 56 is an enlarged scale laterally cut away perspective view of the sear and charger assemblies; 
     FIGS. 57-59 are enlarged scale cut away rear top side perspective views of a specially designed gear system portion of the charger system sequentially illustrating its operation; 
     FIG. 60 is a simplified reduced scale side elevational view of a portion of the gun&#39;s right operating rod showing a gear rack section formed thereon; 
     FIG. 61 is an enlarged scale simplified cross-sectional view through a portion of the charger system taken along line  61 — 61  of FIG. 56; 
     FIG. 62 is an enlarged scale partially cut away rear top side perspective view of a brush housing portion of the charger/sear assembly; 
     FIG. 63 is a highly schematic partial control wiring diagram for the gun; 
     FIGS. 64-67 are partially cut away rear top side perspective views of a barrel lock portion of the gun and sequentially illustrate its operation; 
     FIG. 68 is an exploded perspective view of the components of the barrel lock system; 
     FIG. 69 is an exploded perspective view of a receiver component portion of the gun; and 
     FIG. 70 is an exploded bottom side perspective view of a longitudinal portion of the gun illustrating its rapid and easy disassembly. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIGS. 1-4A, the present invention provides improved armament apparatus  10  which is operatively connected to a representative helicopter  12  (see FIGS. 1-3) having a cabin area  14  positioned rearwardly of a cockpit area  16 . The armament apparatus  10  includes an elongated metal support plank structure  18  which is generally similar to that illustrated and described in U.S. Pat. No. 5,419,234 to Sanderson which has been incorporated herein by reference. 
     The support plank structure  18  is longitudinally extended transversely through cabin area  14  and has a central longitudinal portion  20  which is suitably anchored to the floor  22  of the cabin area  14 , thus operatively securing armament apparatus  10  to the helicopter  12 . Left and right outer end portions  26  and  28  of the support plank  18  (as viewed from the rear of the helicopter  12 ) project outwardly from opposite sides of the helicopter body. Outer plank end portions  26  and  28  have removable outer tip sections  26   a , 28   a  (shown in phantom in FIGS. 2 and 3) which are pivotable, about hinge lines  30  (see FIG.  4 ), relative to their associated plank portions  26  and  28 . 
     When the armament apparatus  10  is utilized, the plank tip sections  26   a , 28   a  are removed, being shown in phantom in FIGS. 2 and 3 for reference purposes only. Mounting tab pairs  32  (see FIG. 4A) are formed on the outer ends of the plank portions  26  and  28  and, using suitable bolts, ball pins or expansion pins (not shown), are securable to corresponding tab pairs (also not shown) on the tip sections  26   a , 28   a  to attach these tip sections as illustrated and described in U.S. Pat. 4,966,063 to Sanderson et al. 
     Turning now to FIGS. 4 and 4A, in addition to the support plank  18  the armament apparatus  10  also includes below each opposite plank end portion  26  and  28  (1) a machine gun, representatively a 30 mm machine gun  34 ; (2) a mounting structure operative to secure the gun  34  to its associated plank end portion  26  or  28  and including specially designed cradle and mount structures  36  and  38 ; and (3) an ammunition feed structure  40 . 
     Armament apparatus  10  further includes (4) a pair of 30 mm magazine boxes  42  secured to the top side of the central plank portion  20  and containing belted  30  mm ammunition (not shown); (5) a pair of elongated flex chute structures  44  through which the belted ammunition passes outwardly from its associated magazine box; and (6) a pair of internally rollered left and right feed adapters  46   a , 46   b  respectively interconnected between the left flex chute structure  44  and the left magazine box  42  (as viewed from the rear of the helicopter), and interconnected between the right flex chute structure  44  and the right magazine box  42 . 
     Magazine boxes  42  are similar to those illustrated and described in U.S. Pat. No. 5,419,234 except that they are sized to accept 30 mm belted ammunition. Similarly, internally rollered feed adapters  46   a , 46   b  are similar to those illustrated in U.S. Pat. No. 5,419,234 but are sized and shaped to handle 30 mm belted ammunition. Flex chutes  44  are of a conventional construction. 
     Each 30 mm machine gun  34 , as shown in FIG. 5, has a horizontally elongated, generally rectangularly cross-sectioned receiver or body portion  48 , a charging and sear assembly housing  50  mounted on the underside of the receiver  48 , a forwardly projecting barrel  52  with a muzzle brake  54  on its forward end and a firing axis A, and a blast suppressor tube  56  (see FIG. 4) coaxially circumscribing and projecting forwardly beyond the muzzle brake. Elongated recoil housings  58  are secured by screws  60  to opposite sides of the receiver  48  and extend longitudinally in front-to-rear directions along their lengths. For purposes later described herein, elongated slots  62  extend through the top and bottom side walls  64 , 66  of the recoil housings  58  (see FIG. 13) adjacent their rear ends. 
     Additionally, along their lengths, the recoil housings  58  have outer side walls  68  joined to their top and bottom walls  64 , 66  by sloping outer top and bottom corner wall portions  70 , 72 . Top corner wall portions  70  slope downwardly and outwardly at a 45 degree angle, and bottom corner wall portions  72  slope upwardly and outwardly at a 45 degree angle. Resilient recoil assemblies (not shown) are operatively disposed within the interiors of the recoil housings  58 . 
     Turning now to FIGS. 6-9 and  13 , each cradle  36  includes a laterally spaced pair of parallel left and right elongated support rail members  74  and  76  that longitudinally extend in front-to-rear directions. Each support rail member  74  and  76  has a generally vertically oriented outer side wall  78 , a top edge wall  80  that slopes upwardly and inwardly toward the other support rail member at a 45 degree angle, and a bottom edge wall  82  that slopes downwardly and inwardly toward the other support rail member at a 45 degree angle. For purposes later described herein, rectangular cutout areas  84  are formed in each of the support rail edge walls  80  and  82  adjacent their front and rear ends. 
     At their front ends the support rail members  74 , 76  extend through and are welded to opposite inner side surface portions of a forward ring structure  86 . As later described herein, the forward ring structure  86  is used as a bore sighting gimbal ring. Rear end portions of the support rail members  74 , 76  extend through and are welded to opposite inner side surface portions of an aft ring structure  88 , with rear end portions of the support rail members  74 , 76  extending rearwardly past the ring structure  88  as shown in FIGS. 6 and 7. An intermediate half ring structure  90  is positioned between the ring structures  86 , 88  and has opposite upper end portions thereof welded to outer side surface portions of the support rail members  74 , 76 . 
     Each of the support rail members  74  and  76  has, along its length, a spaced pair of upwardly projecting feeder attachment pads  92  secured thereto, and an electrical connection box  94  is suitably secured to an outer side portion of the right support rail member  76  adjacent its front end. As illustrated in FIGS. 6 and 7, apertured mounting bosses  96  project outwardly from outer side surface portions of the sloping top and bottom edge walls  80 , 82  of the support rail members  74  and  76  at the rectangular cutout areas  84  thereon. Bosses  96  are used to mount adjustable eccentric cam roller assemblies  98  at the cutout areas  84 . 
     As best illustrated in FIGS. 8 and 9, each of the eight cam roller assemblies  98  used in each cradle structure  36  includes a roller member  100  having a threaded attachment stud  102  centrally secured thereto, a cylindrical bushing  104  with an eccentrically positioned circular opening  106  extending therethrough, a flat washer  108 , and a nut  110 . Each cam roller assembly  98  is attached to its associated support rail mounting boss  96  by positioning the bushing  104  in the opening of the boss  96 , extending the stud  102  outwardly through the bushing opening  106 , and then securing the washer and nut  108 , 110  to the outer end of the threaded stud  102 . 
     This positions the roller member  100  at an associated one of the support rail member cutout areas  84  in a manner permitting the roller member  100  to be laterally adjusted to cause a side portion  100   a  of the roller member  100  to project a selectively variable distance inwardly through its associated cutout area  84  and past the inner side surface of its associated support rail member (see FIGS.  6  and  13 ). Such adjustment of the roller member  100  relative to its associated support rail member is achieved by appropriately rotating the roller assembly boss  104  in the opening of its mounting boss  96 , and then locking the rotationally adjusted boss  104  in place using, for example, a set screw (not shown). This adjustment of the roller members  100 , as best shown in FIG. 13, laterally shifts their rotational axes  112  selected distances inwardly or outwardly, as indicated by the double-ended arrows  114 , thereby correspondingly shifting the inwardly projecting roller member side portions  100   a  inwardly or outwardly relative to the inner side surfaces of their associated support rail member top and bottom edge walls  80  and  82 . 
     With reference now to FIGS. 6 and 7, a spaced pair of upwardly projecting apertured mounting bosses  116  are formed on the top side of the aft ring structure  86 , and a pair of outwardly projecting apertured mounting bosses  118  are formed on its opposite vertical side portions. Each of the bosses  118  extends through a corresponding vertically spaced pair of outwardly projecting apertured bosses  120  formed on rear end portions of the support rail members  74  and  76 . 
     Positioned at the aft end of each of the support rail members  74 , 76  is a pin arm member  122  having a vertically spaced pair of top and bottom flanges  124 , 126  with apertures  128 , 130  in their outer ends. Pins  132 , 134  are respectively extendable through the vertically aligned flange opening pairs  128 , 130 . Each pin arm member  122  is mounted on its associated rear support rail end portion by positioning the flange apertures  128  over the apertures in the mounting bosses  120 , and then extending the pin  132  downwardly through the aligned openings in the flanges  124 , 126  and the openings in the bosses  118  and  120  as best shown in FIG.  6 . 
     On each of the cradle structures  36  the two pins  134  extend through the flange apertures  130  in their associated pin arm member  122 . Also, as schematically indicated in phantom in FIG. 5, the pins  134  extend vertically through the elongated slots  62  in the recoil housings  58  on the opposite sides of the receiver  48  of the machine gun  34  which longitudinally extends through the cradle structure  36  between its left and right support rail members  74 , 76  as illustrated in simplified form in FIG.  13 . 
     As shown in FIG. 13, outer side portions of the opposite pair of recoil housings  58  are complementarily received in the support rail members  74  and  76 , with the suitably adjusted inwardly projecting roller portions  100   a  rollingly engaging the sloping top and bottom corner wall portions  70 , 72  of the recoil housings  58 . This rolling engagement and support of the machine gun  34  within the cradle structure  36  serves to preclude appreciable lateral movement of the supported gun  34  relative to the cradle structure  36  while at the same time permitting the gun  34  to freely move longitudinally along the cradle structure interior during firing recoil and counter recoil motion of the gun, to the extent permitted by the pins  134  that extend through the recoil housing slots  62  as schematically illustrated in FIG.  5 . 
     Referring now to FIG. 10, the ammunition feed structure  40  has a generally rectangular configuration, with elongated rectangular ammunition feed openings  136  and  138  being respectively formed in horizontal top wall and vertical side wall portions  140 , 142  of the feed structure. Each of the two feed structures  40  is secured to the top side of an associated cradle structure  36  by means of bolts  144  extending through the feeder attachment pads  92  into bottom corner portions of the ammunition feed structure  40 . In a manner subsequently described herein, the feed structures function to supply belted ammunition from the magazines  42  (see FIGS. 4 and 4A) to the machine guns  34  supported as described above within the cradle structures  36 . 
     The mount portion  38  of each of the two overall gun mounting structures is shown in FIGS. 11 and 12 and has a generally inverted U-shaped configuration defined by a generally rectangular top deck plate  146  and left and right side plates  148 , 150  respectively depending from left and right side edge portions of the deck plate  146 . Elongated rectangular ammunition feed openings  152 , 154  are respectively formed in aft portions of the top deck plate  146  and the right side plate  150 . As can be seen in FIG. 12, a front bulkhead plate  156  extends between forward end portions of the side plates  148  and  150 . 
     Each of the two mount structures  38  is hung from one of the outer plank end portions  26 , 27  at its hinge line  30  (see FIGS. 4 and 4A) using a hinge line adapter structure  158  bolted to the top side of the deck plate  146  and including two spaced pairs of upwardly projecting tabs  160 . Each tab pair  160  is interdigitated with a corresponding tab pair  32  on the associated plank end (see FIG. 4A) and releasably secured thereto using suitable retaining pin members  162  (see FIG. 4) extended through the interdigitated tab pairs. An alternate embodiment  38   a  of the mount structure  38  is illustrated in FIG.  11 A and is identical to the mount structure  38  with the exception that the hinge line adapter structure  158  is replaced with a pair of bomb lug adapter structures  164  that may be removably secured to corresponding conventional bomb rack structures (not shown herein) mounted on the plank end portions  26  and  28 . 
     The mount structure  38  is secured to the cradle structure  36 , over the feed structure  40  (see FIG.  10 ), at forward and aft portions of the mount structure  38  in a manner which will now be described with continuing reference to FIGS. 11 and 12. Lower forward corner portions  166  of the side plates  148 , 150  are positioned outwardly over opposite left and right side portions of an outer gimbal ring member  168  and secured thereto by bolts  170  which extend inwardly through openings in the corner portions  166  and are threaded into the ring  168 . Bolts  170  are rotatably received in the corner portion openings in a manner permitting the outer gimbal ring  168  to rotate relative to the mount structure  38  about a horizontal axis  172  as indicated by the double-ended arrows  174  in FIGS. 11 and 12. 
     The forward cradle end ring  86 , as best illustrated in FIG. 12, serves as an inner gimbal ring and is received within the outer gimbal ring portion  168  of the mount structure  38 . A diametrically opposite pair of bolts  176  extend inwardly through openings in top and bottom portions of the outer gimbal ring  168  and are threaded into top and bottom side portions of the inner ring  86 . Bolts  176  are rotatable within their associated outer ring openings and thus permit the inner gimbal ring  86  to rotate relative to the outer ring  168  about a vertical axis  178  as indicated by the double-ended arrows  180  in FIG.  12 . 
     Inner and outer gimbal rings  86  and  168 , as described above, permit the cradle  36  (and thus the machine gun  34  supported by the cradle  36 ) to be rotated relative to the mount structure  38  about the horizontal and vertical axes  172 , 178  at a forward end of the mount structure  38  to thereby selectively adjust the gun bore sight elevation and azimuth angles. These elevation and azimuth angles may be selectively adjusted and then locked in by means of a specially designed bore sight adjustment subassembly  182  which is supported on an aft end portion of the mounting structure top deck plate  146  and interconnects aft end portions of the mounting structure  38  and cradle  36  as will now be described in conjunction with FIGS. 14 and 14A. 
     The bore sight adjustment subassembly  182  is of a quite simple construction, comprising only two primary parts—(1) an upper bracket member  184 , and (2) a lower bracket member  186 . The upper bracket member  184  has a generally rectangular body portion  188  which is elongated in a left-to-right direction and has downwardly thickened left and right end portions  190 , 192  with end surfaces  194  and  196 . Slots  198 , 200  are elongated in left-to-right directions and pass in front-to-rear directions through the end portions  190 , 192 . Additionally, a slot  202  extends downwardly through a longitudinally central portion of the body  188  and is elongated in a left-to-right direction. 
     Upper bracket member  184  is positioned in a top rear end cutout area  204  of the mount structure  38  between rearwardly projecting top left and right corner portions  206 , 208  of the side plates  152  and  150 , and is pivotally secured to the corner portions  206 , 208  by bolts  210 , 212  extending through the corner portions  206 , 208  and threaded into openings  214  extending inwardly into the opposite end surfaces  194 , 196  of the body  188 . A pair of rearwardly projecting apertured retaining ears  216  are formed on the left and right body end portions  190 , 192  and have adjustment bolts  218  extending horizontally inwardly therethrough and provided with jam nuts  220  on the outer sides of the ears  216 . A larger adjustment bolt  222  passes downwardly through the top side body slot  202 , through a flat washer  224 , and is threaded through a jam nut  226 . 
     The lower bracket member  186  has a generally U-shaped configuration defined by a generally rectangular body portion  228  elongated in a left-to-right direction and having an upwardly thickened longitudinally central section  230 , and laterally spaced front and rear support plate pairs  232 , 234  projecting upwardly from opposite end portions of the body  228 . Vertically elongated slots  236 , 238  are respectively formed in the front and rear support plates  232  and  234 , and adjustment ears  240  project rearwardly from upper portions of the rear support plates  234 . Projecting downwardly from each of the opposite ends of the lower bracket member body  228  are longitudinally spaced apertured mounting tab pairs  242 , 244 . 
     Lower bracket member  186  is pivotally secured to an aft end portion of the cradle structure  36  by passing the aft ring structure mounting bosses  116  (see also FIGS. 6 and 7) upwardly into the spaces between the downwardly projecting lower bracket member tab pairs  242 , 244  (see FIG. 14) and then passing connecting bolts  246  inwardly through apertures in the tabs  242  and bosses  116  and threading the inner ends of the bolts  246  into threaded openings  248  in the tabs  244  (see FIG.  14 A). The opposite end portions  190 , 192  of the upper bracket member  184  are slidably received between the lower bracket member front and rear support plate pairs  232 , 234  in a manner permitting relative horizontal and vertical sliding movement between the nested upper and lower bracket members  184 , 186 . 
     Still referring to FIGS. 14 and 14A, the nested upper and lower bracket members  184 , 186  are releasably retained in selectively variable vertical and left-to-right horizontal orientations relative to one another by the vertical adjustment bolt  222 , the horizontally oriented adjustment bolts  218 , and a pair of horizontally oriented adjustment bolts  250 . The lower end of the vertical bolt  222  is threaded into an internally threaded opening  252  in the central section  230  of the lower bracket member body  228  and is locked in its rotational orientation by the jam nut  226 , and the inner ends of the adjustment bolts  218  bear against the outer sides of the ears  240 , with the bolts  218  being locked in their rotational orientations by their associated jam nuts  220 . The adjustment bolts  250  forwardly pass sequentially through the vertical support plate slots  238 , the horizontal upper bracket member slots  198 , and the vertical support plate slots  236 , and are threaded into nuts  254  positioned on the front sides of the lower bracket member front support plates  232 . 
     It can be seen that with the bolts  218 , 222  and  250  loosened the lower bracket member  186  may be vertically moved relative to the upper bracket member  184 , thereby correspondingly moving an aft portion of the cradle structure  36  vertically relative to an aft portion of the mount structure  38 . This, in turn, causes the cradle structure  36  (and thus the machine gun  34  supported therein) to pivot about the horizontal front gimbal axis  172  (see FIG. 12) in a manner correspondingly varying the gun&#39;s bore sight elevational angle. Also, the lower bracket member  186  may be moved horizontally in a left-to-right direction relative to the upper bracket member  184 , thereby correspondingly moving an aft portion of the cradle structure  36  horizontally relative to an aft portion of the mount structure  38 . This, in turn, causes the cradle structure  36  (and thus the machine gun  34 ) to pivot about the vertical front gimbal axis  178  (see FIG. 12) in a manner correspondingly varying the gun&#39;s bore sight azimuth angle. 
     As an example, with reference to FIG. 14, to change the gun&#39;s azimuth angle the jam nut  226  and the bolts  218  and  250  are loosened, and the lower bracket member is slid leftwardly or rightwardly as needed relative to the upper bracket member  184 . The jam nut  226  and the bolts  218  and  250  are then retightened to hold the gun in its new azimuth-adjusted position. Similarly, to adjust the elevation angle of the gun, the jam nut  226  and the bolts  218  and  250  are loosened, and the vertical bolt  222  is tightened or loosened in the threaded body opening  252  as necessary to raise the lower bracket member  186  relative to the upper bracket member  184 , or permit the lower bracket member  186  to be moved downwardly relative to the upper bracket member  184 . The jam hut  226  and the bolts  218  and  250  are then retightened to hold the gun in its new elevation-adjusted position. 
     Referring now to FIGS. 4,  4 A,  10  and  11 , in the overall armament apparatus  10  (see FIGS. 4 and 4A) the feed chute adapters  46   a , 46   b  are similar in construction and operation to those illustrated and described in the aforementioned U.S. Pat. No. 5,419,234 but are sized to handle 30 mm ammunition. The left feed chute adapter  46   a  has a generally L-shaped configuration, with its open top end being operatively connected to the outer end of the left flex chute structure  44 , and its open bottom end being coupled to the left feed structure  40  via the top openings  152 , 136  in the left mount structure  38  (see FIG. 11) and the left feed structure  40  (see FIG.  10 ). The right feed chute adapter  46   b  has a generally U-shaped configuration, with its open top end being operatively connected to the outer end of the right flex chute structure  44 , and its open bottom end being coupled to the right feed structure  40  via the side openings  154 , 138  in the right mount structure  38  (see FIG. 11) and the right feed structure  40  (see FIG.  10 ). 
     The unique machine gun mounting structure just described, which serves to support the machine guns  34  on the outer plank end portions  26  and  28 , provides several advantages over conventionally constructed external aircraft machine gun mounting apparatus. For example, the axis-adjustable roller support of the cradle-mounted machine gun  34  firmly precludes appreciable lateral movement of the gun relative to its cradle structure  36 , and thus with respect to the overall mounting structure, while permitting free longitudinal travel of the gun relative to the mounting structure. This precise lateral support of the gun permits the weapon system shown in FIG. 4 to be a “point” weapon system instead of a decidedly less accurate “area” weapon system. 
     Moreover, the pin and slot connection of the cradle structure  36  to its associated gun  34  permits the gun  34 , during firing thereof, to move through a substantially longer recoil and counter recoil stroke than conventional 30 mm machine guns, thereby reducing the maximum firing forces transmitted to the mounting structure and to the support plank. Aiding this recoil force reduction is the unique mounting of the feed structure  40  to the stationary mounting structure instead of on the gun  34  for recoil and counter recoil longitudinal reciprocation therewith. 
     Complementing the firing accuracy improvement obtained by the previously described roller-support of the cradle mounted machine gun is the simplicity and accuracy of the aft-mounted bore sight adjustment structure  182  that, with the forward cradle-to-mount gimbaling system, provides for rapid, easy and reliable firing elevation and azimuth angle adjustment of the gun. 
     While the mounting structure described herein has been representatively illustrated and described in conjunction with 30 mm machine guns, it will be readily appreciated by those skilled in the aircraft armament art that the principles of the present invention could be advantageously applied to other types of machine guns as well if desired. 
     Gun Structure and Operation 
     The basic structure and operation of one of the guns  34  will now be generally described primarily with reference to FIGS. 15-44 which sequentially illustrate the movement of the gun components through a single feed and firing cycle of the gun. As used herein with respect to components of the gun, the terms “left’ and “right” are as viewed from the rear of the gun with the top side of the gun facing upwardly. 
     Generally, the machine gun  34  is of a gas-operated, electrically fired construction. Referring initially to FIGS. 15 and 16, in addition to the previously mentioned components thereof the gun  34  includes, generally from its aft end to its forward end, a bolt unit  260 ; opposing left and right pivotally mounted switch plates  262  (only the right one being shown in the drawings) each surrounded by a generally parallelogram-shaped cam track  264 ; a side-by-side pair of hollow left and right operating rods  266   a  and  266   b  (only the right operating rod  266   b  being visible in FIG.  15 ); a rotatable feed drum  268  mounted in a lower forward portion of the feed structure  40  above the barrel  52 ; and a sear assembly  270  carried in a front end portion of the charger and sear assembly housing  50  below the barrel  52 . Immediately rearwardly of the inner end portion  52   a  of the barrel  52 , which forms the chamber of the gun, is a breech portion  272  of the overall gun body structure which carries the barrel  52 . 
     Turning now to FIGS. 18 and 45, the bolt unit  260  includes a vertically elongated bolt member  274  having an upwardly projecting vertical portion  276  and a rearwardly projecting horizontal portion  278 , and a bolt face member  280  carried on the bolt member portion  276  for forward and rearward movement therewith and vertical movement relative thereto. Left and right side rollers  282  are carried on the bolt face member  280 . The rear ends of the hollow left and right operating rods  266   a  and  266   b  are anchored to an operating rod body structure  284  that underlies the horizontal portion  278  of the bolt member  274 . A top side portion of the body structure  284  has a cutout area  286  with opposing upper and lower cam surfaces  288 , 290  that are rearwardly and upwardly inclined. A forwardly facing vertical ledge surface  292  formed on the bolt member portion  278  opposes a rearwardly facing abutment surface  294  on the operating rod body structure  284 . 
     As best illustrated in FIGS. 25-27, the bolt member  274  is permitted to move forwardly and rearwardly relative to the receiver  48 , but is precluded from vertical movement relative thereto by opposite side tongue portions  296  formed on the horizontal bolt member portion  278  (see FIG. 45 also) and slidingly received in corresponding horizontally extending interior receiver grooves  298 . In a similar manner, the operating rod body structure  284  is permitted to move forwardly and rearwardly relative to the receiver, but is precluded from vertical movement relative thereto, by opposite side tongue portions  300  formed on the body structure  284  and slidingly received in corresponding horizontally extending interior receiver grooves  302 . 
     For purposes later described herein, a generally rectangular secondary mass member  304  (see FIG. 25) is secured to the rear side of the operating rod body structure  284  by shoulder bolts  306  (only one of which is visible) which permit the secondary mass member to slide forwardly toward and rearwardly away from the rear side abutment surface  284   a  of the body structure  284  respectively into and out of contact with the rear side surface  284   a . A spring structure  307  (schematically shown in FIG. 24) resiliently biases the secondary mass member  304  rearwardly away from the body structure abutment surface  284   a.    
     During portions of each feed and firing cycle of the gun  34  as later described herein, the bolt member  274  is releasably locked to the breech  272 , and the operating rod body structure  284  is releasably locked to the bolt member  274 , by means of a lock member  310  (see FIGS. 18 and 45) which is captively retained by the bolt member  274  for vertical movement relative thereto. The lock member  310  is of a generally U-shaped configuration (see FIG.  45 ), and has vertically extending left and right upper portions  312  that slidably extend through left and right cutout areas  314  in the rearwardly projecting lower bolt member portion  278 , and a horizontally extending lower portion  316  that passes through the cutout area  286  in the operating rod body structure  284 . As later described herein, parts of the vertical portions  312  of the lock member  310  are releasably interlockable with corresponding breech recesses, and the lower lock member portion  316  has (as best illustrated in FIG. 22) sloped top front and bottom rear corner surfaces  288   a , 290   a  which are respectively parallel to the upper and lower cam surfaces  288 , 290  on the cutout area  286  in the operating rod body structure  284 . 
     With reference now to FIGS. 5,  15  and  16 , immediately forward of the receiver  48  the gun body is provided with a top side feed cover plate  318  in which an elongated cam slot  320  is formed. Cam slot  320  has a front recoil dwell or “overrun” portion  320   a  which is parallel to the axis A, a central drive portion  320   b  which is angled relative to the axis A, and a rear counter-recoil dwell portion  320   c  which is parallel to the axis A. A drive pin  322  projects downwardly from the rotatable feed drum  268  and is slidably received in the cam slot  320 . Via a ratcheted gear train generally denoted at  324 , the feed drum  268  is drivingly connected to the forward end of a splined drive shaft  326  that is positioned above the drum  268  and longitudinally extends parallel to the gun axis A. 
     Shaft  326  is coaxially anchored to an axially spaced pair of ammunition feed sprockets  328  (which are more completely illustrated in FIGS. 37-39) used to operatively draw cartridges  330  into and through the feed structure  40  as later described herein. Cartridges  330  are held in conventional belt form, for passage through their associated flex chute  44  (see FIG. 4) by a series of detachable link members  332 . Representative cartridges  330   a ,  330   b ,  330   d ,  330   e  and  330   f  are shown in FIGS. 15 and 16, with cartridge  330   a  being the next cartridge to be chambered and cartridge  330   c  having been removed from between cartridges  330   b  and  330   d  for illustrative clarity. As later described herein, during each feed and firing cycle of the gun  34 , the drum  268  is first rotated in a back-indexing direction, during which time the ratcheted gear train  324  does not rotate the sprocket shaft  326 , and is then rotated in the opposite direction to rotate the sprockets  328  in a feed direction. 
     For ammunition handling purposes later described herein, a pair of generally arcuate spring-loaded ejector members  334  (see FIGS. 18 and 45) are mounted on a front part of the bolt face member  280  on opposite sides thereof for pivotal movement outwardly from and inwardly toward the bolt face member  280 . A pair of spring-loaded extractor members  336  (see FIGS. 24,  25  and  45 ) are carried on opposite sides of the bolt member  274  behind the ejector members  334 . 
     A side-by-side pair of elongated guide rods  338  (see FIG. 17) are anchored at their rear ends to a pair of mounting blocks  340  secured to a lower rear end portion of the receiver  48 . From the blocks  340  the guide rods  338  longitudinally extend forwardly through openings in the operating rod body structure  284  and coaxially through the interiors of the left and right operating rods  266   a  and  266   b . Coiled drive spring members  342  circumscribe the guide rods  338  and also extend through the interiors of the operating rods  266   a ,  266   b . The drive spring members  342  bear at their rear ends against the mounting blocks  340 , and at their front ends against front end cap portions  344  of the operating rods  266   a , 266   b  (see FIGS.  30  and  32 ), thereby resiliently biasing the operating rods  266   a , 266   b  (and thus the operating rod body structure  284 ) in a forward direction. 
     During operation of the gun  34  as later described herein, the left and right operating rods  266   a , 266   b  are forwardly and rearwardly movable coaxially through the interiors of a side-by-side pair of extension tubes  346  having front ends that open into the interior of a hollow front end unit  348  (see FIGS. 30 and 32) disposed on a lower front end portion of the gun body. As best illustrated in FIG. 30, rearwardly adjacent the front end of the feed cover plate the gun barrel  52  has a circumferentially spaced series of side wall gas outlet ports  350  that communicate the interior of the barrel  52  with an annular plenum  352  formed on the outer side of the barrel  52  and communicating on a lower side portion thereof with the interior of the hollow front end unit  348 . Along a front end portion thereof, each of the two extension tubes  346  is provided with an annularly spaced series of side wall gas vent openings  354  (see FIGS.  30  and  32 ). 
     Referring briefly now to FIG. 19, the sear assembly  270  includes a sear solenoid  356  having a vertically movable elongated plunger portion  358  which is upwardly biased toward an “on-sear” or pre-firing position (see FIGS. 15 and 16) by a coil spring member  360 . With the sear plunger  358  in such on-sear position, cartridge chambering in the gun is precluded. As later described herein, when the solenoid  356  is energized, the plunger  358  is driven downwardly to its FIG. 19 “off-sear” position (see also FIG.  17 ), against the resilient resistance of the spring  360 , chambering of successive cartridges  330  (i.e., operative positioning of the cartridges in the rear end portion of the barrel) in conjunction with the overall feed and firing cycles of the gun is permitted. 
     With initial reference to FIGS. 15 and 16, the basic firing cycle of the gun  34  will now be described. Subsequent to this operational description, various previously mentioned structures and assemblies, and other unique features of the gun  34  will be described in greater detail. 
     The gun  34  is shown in FIGS. 15 and 16 in its stationary, ready-to-fire open bolt position, with no cartridge chambered, in which the gun is at rest in its forward position within the previously described cradle structure  36 . Bolt unit  260  is “on sear” and in its rearwardmost, open bolt position. The sear plunger  358  is in its upper “on-sear” position and functions as later described herein to hold the left and right operating rods  266   a ,  266   b  in rearwardly retracted positions thereof, against the resilient biasing force of the guide rod springs  342  (see FIG.  17 ), with the operating rod body structure  284  rearwardly abutting the mounting blocks  340 . 
     The bolt face rollers  282  (see FIGS. 18 and 45) are disposed within rear end portions of the horizontal bottom sections  264   a  of the switch plate cam paths  264 , and the bolt face member  280  is in its lowermost position on the bolt member  274 . Along its front side the bolt member  274  has a generally conventional “T-bolt” configuration, with a spaced pair of left and right vertical extractor grooves  361  being formed in a front side portion of the bolt member  274  as best illustrated in FIG.  45 . Opposite left and right side portions of the rear end casing rim portion  364  of the initial cartridge  330   a  (see FIG. 24) are captively and slidably retained in the integral extractor grooves  361  on the bolt member  274 , with the cartridge  330   a  being ready to be forwardly moved into the chamber  52   a , and the ready cartridge  330   b  (i.e., the next cartridge to be chambered) being positioned to be captured by the bolt during the subsequent feed and firing cycle of the gun as later described herein. Bolt face ejectors  334  are held laterally inwardly against the cartridge  330   a , above and slightly forwardly of the integral extractor grooves  361 , by facing left and right interior side surface portions of the receiver  48 . 
     In this initial position of the gun  34  the feed cycle is static, and the feed drum  268  has rotationally moved to its maximum feed rotation position, with the depending peripheral pin portion  322  of the feed drum being received in the straight counter-recoil “overrun” portion  320   c  of the feed cover plate cam slot  320  (see also FIG.  5 ). The lock member  310  is in its lowermost position, and is thus not locking the bolt member  274  to the breech  272 . In its lowermost position the lock member  310  is, however, interlocking the operating rod body structure  284  to the bolt member  274  in a manner such that forward movement of the body structure  284  relative to the gun body will correspondingly move the bolt member  274  in a forward direction relative to the gun body. 
     Referring now to FIGS. 17-20, upon initiation of firing of the gun  34  electrical power is transmitted to the sear solenoid  356  (see FIG. 19) in a manner causing the plunger  358  to downwardly driven to its “off-sear” position which, as previously mentioned, releases the left and right operating rods  266   a and  266   b , and their associated body structure  284 , and permits the body structure  284  to be forwardly driven away from the mounting blocks  340 , by the guide rod springs  342 , as may be seen by comparing FIGS. 17 and 20 to FIGS. 15 and 16. This, in turn, forwardly drives the operating rods  266   a  and  266   b  into the interiors of their extension tubes  346 . 
     Via the lock member  310 , the spring-driven forward movement of the overall operating rod structure  266 , 284  also forwardly moves the bolt unit  260  relative to the balance of the gun, with the bolt face rollers  282  (see FIG. 18) reaching the forward ends of their associated lower horizontal switch plate cam path portions  264   a  and approaching forwardly and upwardly sloped front portions  264   b  of the cam paths  264 . Since the bolt face rollers  282  are still in their associated horizontal lower switch plate cam track portions  264   a , the bolt face member  280  remains in its lowermost position on the bolt member  274 . 
     During this initial portion of the gun&#39;s firing cycle only the operating rod structure  266 , 284  and the bolt unit  260  are moving—the balance of the gun  34  is still in its forward stationary battery position within the cradle structure  36 . The feed cycle is still static, since the feed drum  268  has not been rotated because the gun  34  is not moving, and the feed drum pin  322  is still in the feed cover plate cam slot portion  320   c . As illustrated in FIGS. 17,  18  and  20 , during this initial portion of the firing cycle the bolt-captured cartridge  330   a  is being driven into the chamber  52   a  by the forwardly moving bolt member  274 . 
     The bolt face member  280  is ready to be cammed upwardly along the bolt member  274  when the bolt face rollers  282  forwardly traverse the upwardly and forwardly sloped switch plate cam track front end portions  262   b . Additionally, the ejectors  334  are still precluded from moving laterally outwardly by opposing left and right interior side surface portions of the receiver, but are forwardly approaching left and right side breech clearance cutout areas  362  (the right breech cutout area  362  being visible in FIG. 18) which will permit the ejectors  334  to swing outwardly therethrough and out of operative engagement with the captured cartridge  330   a.    
     During further spring-driven forward movement of the operating rod structure  266 , 284  and bolt unit  260  relative to the still stationary balance of the gun  34  the captured cartridge  330  is nearly chambered as illustrated in FIGS. 21 and 22, the bolt face rollers  282  (see FIG. 22) are moving upwardly and forwardly along their switch plate cam track portions  264   b  to correspondingly move the bolt face member  280  upwardly along the forwardly moving bolt member  274 , and the ejectors  334  have swung laterally outwardly into the facing left and right breech cutout areas  362  to permit the bolt face member  280  to move upwardly in preparation for capture of the ready cartridge  330   a  by the bolt unit  260 . Since the gun  34  has still not moved relative to its associated cradle structure  36 , the ammunition feed cycle is still static, with the feed drum drive pin  322  still being received in the rear feed cover plate cam slot portion  320   c.    
     As the bolt face rollers  282  are moving up their associated switch plate cam track portions  264   b  they pivot the spring-loaded switch plates  262  in a clockwise direction as viewed in FIG.  22 . When the rollers  282  reach the upper ends of the cam track portions  264   b , the spring-loaded switch plates  262  snap back to their original positions in a manner causing the rollers  282  to traverse their horizontal upper cam track portions  264   c  during the subsequently described rearward movement of the bolt unit  260 . As the captured cartridge  330   a  is being chambered, the operating rod end caps  344  (see FIG. 21) are about to enter the hollow gas piston area of the front end unit  348 . The balance of the gun  34  is still stationary relative to its associated cradle structure  36 . Lock member  310  is still in its lowermost position and has not yet locked the bolt member  274  to the breech  272 . 
     Turning now to FIGS. 23-25, during continued spring-driven forward movement of the bolt unit  260  the bolt unit further chambers the captured cartridge  330   a  and begins to capture the ready cartridge  330   b  as the bolt face rollers  282  move forwardly through their upper switch plate cam track portions  264   c . The gun  34  is still stationary, and in its forward battery position within the cradle structure  36 , and the feed cycle is still static. Additionally, the front end cap portions of the operating rods  266  are forwardly entering the interiors of subsequently described cylinder portions of the front end unit  348 . Cartridge  330   a  has been almost chambered, and the bolt face ejectors  334  are now back in their “down” positions around a rear end portion of the ready cartridge  330   b  and are again held from swinging outwardly by side portions of the receiver above the opposing left and right breech clearance cutout areas  362 . 
     Additionally, as best illustrated in FIG. 25, the extractors  336  are being cammed outwardly by the rear end casing rim  364  of the ready cartridge  330   b  in the feeder in preparation to snap into the annular casing groove  366  immediately forward of the rim  364 . For purposes of illustrative clarity the left extractor  336  is shown in FIG. 25 without the supporting structure that mounts it on the bolt member  274 . The forwardly moving bolt face unit  280  is still rearwardly spaced apart from a bolt sensor electrical switch  368  (see FIG.  26 ), located on the rear face  370  of the sprocket support structure  371 , which the bolt face unit  280  will later forwardly engage and depress to close a portion of an overall electrical firing circuit as later described herein. Additionally, with the gun components in their relative positions shown in FIGS. 23-25, the lock member  310  is still in its lowermost position in which it latches the operating rod body structure  284  and the bolt member  274  together for conjoint forward movement but has not yet locked the bolt member  274  to the breech  272 . 
     Next, as illustrated in FIG. 26, the forwardly moving bolt face member  280  contacts, depresses and closes the bolt sensor switch  368 , with an ogive bumper structure  372  operating to limit the forward travel of the ready cartridge  330   b  in the feeder. As can be seen from the foregoing portion of the overall operational sequence of the gun  34 , the bolt sensor switch  368  can only be closed after the bolt unit  260  has captured the new ready cartridge  330   b.    
     At the FIG. 26 stage in the operational sequence of the gun  34  the initial cartridge  330   a  has been fully chambered, and the bolt member  274  has reached the forward limit of its travel relative to the still stationary gun. However, the inertia of the operating rod structure  266 , 284  now carries it forwardly relative to the now stationary bolt unit  260  toward a lower front portion  278   a  of the bolt member  274 . As will now be described, this forward motion of the operating rod body structure  284  relative to the now stationary bolt member  274  causes the lock member  310  to be cammed upwardly toward its breech locking position in which the lock member  310  functions to lock the bolt member  274  to the breech  272 . 
     Referring now to FIGS. 36,  36 A and  45 , each of the opposite vertical portions  312  of the lock member  310  (see FIG. 45) has, on its outer side, outwardly projecting front and rear top lugs  406  and  408 , and outwardly projecting front and rear bottom lugs  410  and  412 . Each of the front bottom lugs  410  has a downwardly and rearwardly sloped front bottom corner surface  414 . As the bolt unit  260  forwardly approaches the left and right breech cutout areas  362  (see FIGS.  36  and  36 A), the top lugs  406 , 408  are received in and slide longitudinally along the opposite top receiver grooves  298 , and the bottom lugs  410 , 412  are received in and longitudinally slide along the opposite bottom receiver grooves  302 , to thereby prevent the lock member from moving upwardly relative to the horizontal portion  278  of the bolt member  274 . 
     Each of the opposing left and right breech cutout areas  362  (see FIG. 36A) has formed therein a forwardly and upwardly inclined starting cam surface  416  at the front end of its associated lower receiver groove  302 ; front and rear bottom lateral recesses  418 , 420  extending vertically between portions of the receiver grooves  298  and  302   p ; and front and rear top lateral recesses  422 , 424  positioned along the top side of the upper receiver groove  298  and respectively above the recesses  418 , 420 . 
     As the bolt unit-carried lock member  310  forwardly reaches the opposing left and right breech cutout areas  362 , the inclined lock member lug corner surfaces  414  (see FIG. 45) contact the breech starting cam surfaces  416  (see FIGS. 36 and 36A) to initiate the upward travel of the lock member  310  relative to the bolt member  274  as the lock member lug pairs  406 , 410  and  408 , 412  respectively come into horizontal alignment with the vertical breech recess pairs  422 , 418  and  424 , 420 . 
     Next, by he interaction between the sloping upper and lower cam surfaces  288 , 290  on the operating rod body structure  284  with the facing surfaces  288   a , 290   a  on the lower lock member portion  316  (see FIG. 26) the lock member  310  is cammed upwardly to its breech locking position in which the lock member lugs  406 , 410  (see FIG. 45) are respectively received in the breech cutout area recesses  422  and  418 , and the lock member lugs  408 , 412  are respectively received in the breech cutout area recesses  424 , 420 . This lug receipt prevents rearward movement of the lock member  310 , and thus the balance of the bolt unit  260 , relative to the breech  272 . 
     As previously mentioned, the bolt sensor switch  368  is a first switch portion of an electrical firing circuit which must be closed in order to fire a chambered cartridge. A second switch portion  374  is disposed in series with the bolt sensor switch  368 , and both switches must be closed to close the firing circuit and permit a chambered cartridge to be fired. 
     Second switch portion  374  includes a metal electrical probe  376  (see also FIGS. 27 and 29) extending upwardly beyond the top side of the operating rod body structure  284 , a metal bolt primer hot shoe  378  positioned on an undercut bottom side section of the bolt portion  278  immediately behind the bolt surface  278   a , and a dielectric plate member  380  extending along the undercut bottom side section immediately behind the hot shoe  378 . Hot shoe  378 , in turn, engages an electrical firing contact structure  382  which operatively engages a central rear casing end portion of the chambered cartridge  330   a . The second switch portion  374  is closed when the electrical probe  376  contacts the hot shoe  378 , and is opened when the probe  376  contacts the dielectric plate  380 . 
     As can be seen in FIG. 26, the forwardly moving electrical probe  376  is forwardly sliding along the dielectric plate  380 , and has not yet engaged the hot shoe  378  to close the second switch  374 . Additionally, with the bolt member  274  in its forward limit position, the bolt face rollers  282  (not visible in FIG. 26) have rolled upwardly onto associated ramp surfaces  384  positioned at the front ends of the horizontal top switch plate cam track portions  264  and slightly elevated with respect thereto. 
     Referring now to FIG. 27, further forward movement of the operating rod body structure  284  relative to the now stationary bolt member  274  brings the body structure  284  and the operating rods  266  to their forward limit position. At this point the forwardly moving operating rod body structure  284  has cammed the lock member  310  upwardly to its uppermost position in which it locks the bolt member  274  to the breech  272 , and the operating rod body structure  284  has hit the bolt portion  278   a  so that now neither the lock member  310  nor the overall operating rod structure  266 , 284  are moving within the receiver. The secondary mass member  304  now slams forward into the stopped operating rod body structure  284  to prevent it from bouncing rearwardly off the bolt member abutment portion  278   a  and potentially unlocking the bolt member  274  from the breech. 
     As previously mentioned, during this forward movement of the operating rod body structure  284  relative to the stationary bolt member  274 , the bolt sensor switch  368  is closed. When the forwardly moving operating rod body structure  284  reaches its FIG. 27 position the probe  376  is brought into contact with the bolt primer shoe  378 , thereby closing the switch  374 , and establishing electrical contact between the operating rod body structure  284  and the bolt member  274 . This closes the overall electrical firing circuit and fires the chambered cartridge  330   a . It should be noted that only until after the lock member  310  is fully cammed to its uppermost breach locking position can the operating rod body structure  284  move far enough forwardly to make electrical contact with the primer shoe  378  and close the switch  274 . This assures that the gun  34  must be fully locked before it can be fired. 
     The firing of the initial cartridge  330   a , as just described, is effected by the closure of the lower switch structure  374 . However, as later described herein, the firing of each subsequent cartridge in a given burst is effected by closure of the bolt sensor switch  368 . 
     FIGS. 28-30 illustrate the gun  34  just after the initial cartridge  330   a  has been fired. By such firing of the cartridge  330   a  the gun  34  is put into “free” rearward recoil motion relative to the cradle structure  36  against the resilient resistance of the previously mentioned recoil structures within the recoil housing portions  58  of the gun  34  (see FIG.  5 ). During this initial free recoil movement of the gun, the bolt unit  260  is still locked up to the breech, the gas operating system has not yet begun to function, and neither the bolt unit  260  nor the operating rod structure  266 , 284  is moving within the receiver  48 . Since the gun  34  is now moving rearwardly relative to the cradle structure  36  and the feeder  40 , the captured ready cartridge  330   b , as best illustrated in FIG. 29, is being rearwardly withdrawn by the bolt unit  260  from the feeder and from the cartridge&#39;s link  332 . 
     The feed drum drive pin  322  is approaching the central angled cam slot portion  320   b , but is still in the straight rear recoil portion  320   c  thereof. Accordingly, the feed drum  268  has still not been rotated away from its initial FIG. 15 position and the feed system is still static. Additionally, the downward unlocking movement of the lock  310  has not yet been initiated, and the fired cartridge  330   a  is still locked up in the chamber  52   a.    
     Referring now to FIG. 30, the front end unit  348  has a pair of hollow cylinder portions  386  disposed on opposite sides thereof, the interiors of the cylinders  386  being communicated with the interior  348   a  of the front end unit  348  by means of a cross-hole  388  extending outwardly from the body of the front end unit  348  into the cylinder portions  386  thereof. At the firing of the cartridge  330   a , the operating rod end caps  344  are slidingly received in their associated front end unit cylinder portions  386 . After the projectile portion of the fired cartridge  330   a  forwardly passes the front end unit  348 , pressurized gas  390  within the barrel  52  behind the projectile traversing the barrel  52  is sequentially forced outwardly from the barrel  52  into the annular plenum  352  through the barrel outlet ports  350 , into the interior  348   a  of the front end unit  348 , and into the cylinders  386  via the cross-hole  388 . 
     As depicted in FIGS. 31 and 32, pressurized propellant gas  390  entering the front end unit cylinders  386  rearwardly drives the operating rods  266  (as indicated by the arrow  392  in FIG. 32) through the extension tubes  346  to rearwardly withdraw the operating rod front end caps  344  from the cylinders. As indicated in FIG. 31, this rearwardly drives the operating rod body structure  284  relative to the still locked-up bolt unit  260  and away from the bolt member portion  278   a  that the body structure  284  previously abutted. As indicated, the spent cartridge  330   a  remains locked up in the chamber  52   a.    
     The gas-driven initial rearward movement of the operating rod structure  266 , 284  is faster than the rearward recoil velocity of the balance of the gun  34 , and serves to begin the unlocking of the bolt member  274  from the breech  272  by causing the operating rod body structure cam surfaces  288 , 290  to engage the corresponding surfaces  288   a , 290   a  on the lock portion  316  and cam the lock member  310  downwardly from its previous uppermost bolt-to-breech locking position. As can be seen in FIG. 31, this initial rearward movement of the operating rod body structure  284  relative to the still locked-up bolt member  274  also opens the lower firing switch structure  374 . Additionally, the pressurized gas  390  entering the front end unit cylinders  386 , which exerts a rearward force on the operating rods  266 , exerts a forwardly directed reactive force on the front end unit  348 , and thus on the gun  34  as well, to thereby offset and slow the rearward recoil movement of the gun  34 . 
     Turning now to FIG. 33, further rearward movement of the operating rod body structure  284  downwardly moves the lock member  310  to its lowermost unlocked position, thereby unlocking the bolt member  274  from the breech and permitting the rearwardly moving operating rod body structure  284  to rearwardly move the bolt member  274  within the receiver in the rearwardly recoiling gun  34 . The rearward movement of the operating rods  266  has moved their front end caps  344  rearwardly past the extension tube vent openings  354  (see FIG. 30) to thereby cause the gas  390  to be vented outwardly through the openings  354 . 
     During this rearward movement of the operating rod body structure  284  and the bolt unit  260  relative to the rearwardly recoiling balance of the gun  34 , several other things happen. Specifically, the bolt face rollers  282  move rearwardly down the ramp surfaces  384  and are then moved rearwardly along the horizontal top switch plate cam track portions  264   c  to their rear ends. For purposes later described herein, the movement of the right bolt face rollers  282  downwardly off their associated ramp surfaces  384  correspondingly moves the bolt face member  280  downwardly on the bolt member  274  a distance of approximately 0.125″. 
     This small downward movement of the bolt face member  280  moves a small stabilizing tooth portion  394  thereon into the rear end casing groove  366  of the ready cartridge  330   b  prior to its rearward extraction from the feeder. Next, as the bolt face rollers  282  move rearwardly along the upper cam track portions  264   c  the rearwardly moving bolt unit  260  (by means of the integral bolt member extractor grooves  361 ) rearwardly extracts the spent casing portion of the fired cartridge  330   a  from the chamber  52   a , positioning it over an open bottom side portion  48   a  of the receiver  48 , and also (by means of the bolt face extractor members  336 ) rearwardly extracts the captured ready cartridge  330   b  from its associated link  332  within the feeder  40 . As indicated in FIG. 33, after the ready cartridge  330   b  has been extracted from its associated link  332  the link is retained on the sprocket members  328  within the feeder  40 . 
     During the continued rearward recoil of the gun  34  shown in FIG. 33 the feed drum drive pin  322  enters and begins to move through the angled central portion  320   b  of the feed cover plate cam slot  320 , thereby rotating the feed drum  268  in a back indexing direction. Due to the ratcheted nature of the drive train  324 , this back indexing rotation of the feed drum  268  does not rotate the splined drive shaft  326  and the sprockets  328  thereon, but merely indexes the feed drum  268  for a subsequent reverse rotation thereof, as later described herein, to operatively rotate the sprockets  328  for the next feed cycle. 
     Referring next to FIG. 34, while the gun  34  continues to rearwardly recoil relative to its associated cradle structure  34 , and the bolt unit  260  continues to move rearwardly within the receiver  48 , the feed drum  268  continues to be rotated in a back indexing direction while its pin  322  remains in the cam slot portion  320   b , and the bolt face rollers  282  enters and are forced downwardly through the downwardly and rearwardly sloped rear end portions  264   d  of their associated switch plate cam tracks  264 . This cams the bolt face member  280  downwardly along the bolt member  274  and correspondingly drives the captured and extracted ready cartridge  330   b  downwardly along the front side of the bolt member  274 . Via the bolt face ejector members  334 , this causes the casing rim  364  of the cartridge  330   b  to be driven downwardly from the extractor members  336  and into the bolt member integral extractor grooves  361  and forces the cartridge  330   b  into engagement with the spent casing of the fired cartridge  330   a  to begin to drive it downwardly off the bolt member  274  (i.e, to drive its casing rim  364  out of the extractor grooves  361 ) for ejection downwardly through the underlying open bottom side portion  48   a  of the receiver  48 . The receipt of the bolt face tooth  394  in the rear end casing groove  366  of the ready cartridge  330   b  helps to keep the cartridge  330   b  from undesirably wobbling and “chattering” as it is forced down the front side of the bolt member  274  and used to eject the spent casing of the fired cartridge  330   a.    
     Next, as illustrated in FIG. 35, with the gun  34  still in rearward recoil travel relative to its associated support cradle structure  36  and the feed drum  268  still being rotated in a counter indexing direction, the bolt unit  260  reaches its rear limit of travel within the receiver  48 . At this point the rear side of the bolt member  274  engages and compresses coiled buffer springs  396  supported on and projecting forwardly from rod portions  398  of a plurality of bolt recoil block members  400  suitably supported rearwardly of the bolt member  274  (see also FIG.  34 ). 
     The bolt face rollers  282  have been moved into the rear ends of the lower switch plate cam path portions  264  after having tilted the switch plates  262  in a clockwise direction as the rollers  282  exited their associated cam path portions  264   d . Upon entry of the rollers  282  into the cam path portions  264   a , the spring-loaded switch plates  262  have snapped back into their original positions to block the re-entry of the rollers  282  into the cam path portions  264   d  from the cam path portions  264   a.    
     The movement of the bolt face rollers  282  into the lower cam path portions  264   a  correspondingly moves the bolt face member  280  to its lower limit position on the bolt member  274 . In turn, the bolt face member  280  moves the ready cartridge  330   b  downwardly into alignment with the chamber  52   a  and causes the ready cartridge  330   b  to downwardly push the spent cartridge  330   a  off the front side of the bolt member  274  so that the spent cartridge  330   a  is downwardly ejected through the open bottom side portion  48   a  of the receiver  48  as indicated in FIG.  35 . Notice that with the operational cycle of the gun  34  in its FIG. 35 stage, the secondary mass member  304  is spring-biased rearwardly away from the opposing rear abutment surface portion  284   a  of the operating rod body structure  284 . 
     After the rearwardly traveling bolt unit  260  strikes and compresses the bolt buffer springs  396  the bolt unit/buffer structure impact rearwardly kicks the gun  34  through the final increment of its recoil travel, as shown in FIG. 36, while at the same time utilizing the compressed guide rod and bolt buffer springs  342  and  396  to cause the bolt unit  260  (and thus the operating rod structure  266 , 284 ) to bounce forwardly, in a counter-recoil direction, relative to the still rearwardly recoiling gun  34  in a counter-recoil direction. At this time the feed drum pin  322  has entered the forward feed cover plate slot portion  320   a , and the feed drum has been fully back-indexed and is ready to initiate the next ammunition feed cycle. Also, as indicated in FIG. 36, the forwardly moving bolt unit  260  is moving the captured cartridge  330   b  forwardly toward the chamber  52   a  with which the cartridge  330   b  is aligned. 
     Next, as indicated in FIG. 37, the gun  34  begins its forward counter-recoil motion (such motion being created by the resilient recoil assemblies disposed in the gun&#39;s recoil housings  58  as later described herein) so that now both the gun and the bolt/operating rod structures are moving forwardly in the counter-recoil direction. Previously ejected spent cartridge  330   a  has been hit by the front side of the forwardly moving operating rod body structure  284  and knocked forwardly and clear of the gun  34 . 
     The forward counter-recoil movement of the gun  34  relative to its associated support cradle structure  36  initiates the ammunition feed cycle by causing the feed drum pin  322  to enter and rearwardly traverse the central angled feed cover plate cam slot portion  320   b  to thereby rotate the feed drum  268  in a feed direction opposite from its previously described back-indexing direction. Rotation of the feed drum  268  in this reversed direction correspondingly rotates the sprockets  328  in their feed direction—i.e., in a clockwise direction as viewed from the front in FIG.  38 . The feed drum index is 120 degrees for 60 degrees of sprocket rotation. From their FIG. 36 position to their FIG. 37 position, the feed sprockets  328  have been rotated 10 degrees in response to a 20 degree driven rotation of the feed drum  268 . This rotationally advances the cartridge  330   c  closer to its “ready cartridge” orientation. 
     In the meantime, the forwardly moving bolt unit  260  has begun to chamber the captured cartridge  330   b . Additionally, the forwardly moving bolt unit  260  has brought the bolt face rollers  282  to the forward end of the lower switch plate cam path portions  264   a  while the bolt face member  280  has remained in its lower limit position on the bolt member  274 . This readies the rollers  282  for entry into their associated front end cam path portions  264   b  to thereby upwardly cam the bolt face member  280  toward its upper limit position on the bolt member  274 . It should be noted that at this FIG. 37 point in the operational cycle of the gun  34  the gun is still well aft of its at rest “in battery” position, while the bolt unit  260  is almost fully forward within the gun and is chambering the new cartridge  330   b.    
     During the next stage of the gun&#39;s operational cycle, shown in FIG. 38, the feed cycle continues, with the feed drum  268  now having been rotated through approximately half of its total 120 degree feed rotation so that the sprockets  328  have correspondingly been rotated approximately 30 degrees. The right bolt face roller  282  is forwardly traversing the inclined front end portion  264   b  of the switch plate cam path  264 , and the bolt face member  280  has been moved approximately half way up the bolt member  274  toward its upper limit position. The cartridge  330   b  is being further chambered, and is now far enough into the chamber  52   a  to permit the bolt face ejectors  334  (not visible in FIG. 38) to move out and over the cartridge  330   c.    
     Turning now to FIGS. 39-41, in the next stage of the gun&#39;s operational cycle the feed drum  268  has been rotated approximately 90 degrees through its 120 degree feed rotation, with the sprockets  328  having been rotated 45 degrees. The bolt unit  260  has reached the limit of its forward travel in the receiver, and the lock member  310  has been cammed upwardly to its uppermost position to thereby lock the bolt member  274  to the breech as previously described. The front side of the operating rod body structure  284  has forwardly struck the lower bolt member portion  278   a.    
     As illustrated in FIG. 39, the lower firing switch structure  374  is now closed. However, the bolt face member  280  (which is now locked to the breech and being moved forwardly by the forwardly moving gun) is still forwardly approaching the bolt sensor switch  368  (not visible in FIG. 39) and has not yet contacted and closed it yet. Accordingly, the gun firing circuit is still open so that the chambered cartridge  330  cannot be fired yet. As illustrated, after the operating rod body structure  284  has forwardly struck the bolt portion  278   a  the secondary mass member  304  forwardly strikes the bottomed-out operating rod body structure  284  to stop undesirable rear kick-back thereof. 
     At about this point in the operational cycle of the gun  34  the receiver  48  forwardly enters the feed structure  40  (see FIG. 40) and is automatically centered therein by feed guide rollers  402  (see FIG. 10 also) which are mounted on the feeder  40  and rollingly engage opposite left and right side portions of the receiver  48  as it forwardly enters the interior of the feeder  40 . As illustrated in FIG. 41, during the feed sprocket rotational increment between the sprocket positions shown in FIGS. 38 and 39 the rotating feed sprockets  328  eject the link  332 , which previously held the now chambered cartridge  330   b , outwardly through a right side opening  404  in the feeder  40 . 
     In the next portion of the gun&#39;s operational cycle, shown in FIGS. 42 and 43, the gun  34  has moved further forwardly through its counter-recoil movement, and the feed cycle is over. The 120 degree feed rotation of the feed drum  268  has been completed, thereby completing the corresponding 60 degree rotation of the sprockets  328 , and the new ready cartridge  330   c  is in a position to be captured by the forwardly moving bolt unit  260  (as previously described) which is locked to the forwardly moving gun  34 . The lower firing switch structure  374  is closed, but the bolt sensor switch  368  (see FIG. 43) has not yet been contacted, depressed and closed by the forwardly moving bolt face  280 . 
     Finally, as indicated in FIG. 44, the forwardly moving bolt unit  260  captures the ready cartridge  330   c  (as previously described with respect to the previous ready cartridge  330   b ), and then reaches its forward limit position as it depresses the bolt sensor switch  368  (see FIG. 43) to thereby fire the first shot of the gun  34  in its automatic mode during this particular burst (the first shot having been fired, as previously mentioned, with the gun in its open bolt mode). 
     Because the gun in this automatic post-first shot mode is fired while it is still moving into the feeder, the initial firing impulse must first stop the gun&#39;s forward movement before gun recoil travel to the rear can start. This “out of battery” firing reduces the cartridge impulse the gun must account for by approximately twenty percent, thereby providing a substantial reduction in the gun recoil load. 
     During each given firing burst, the sear solenoid  356  is maintained in an energized state that keeps the sear plunger  358  in its down position to permit the operating rod structure  266 , 284  to be cycled back and forth as previously described. When the solenoid  356  is de-energized to end the firing burst the sear plunger  358  returns to its upper limit position and acts to stop the operating rod structure  266 , 284  in its FIG. 15 rear “open bolt” position. 
     Turning now to FIG. 46, the previously mentioned front and rear ammunition feed sprockets  328  are mounted on a support structure  371  that includes a hollow tubular hub member  426  having opposite ends to which the sprockets  328  are secured. The interior surface of the hub  426  is grooved to slidingly receive a rear end of the splined drive shaft  326 , thereby permitting the hub  426  (and thus the sprockets  328 ) to move forwardly and rearwardly along the drive shaft  326  while remaining locked to the shaft so as to be rotationally drivable thereby. The axial movement of he hub  426  along the rear end of the splined shaft  326  is limited by a transverse stop pin  428  extending through the hub  426  and being received in a longitudinal side surface groove  430  in a rear end portion of the splined drive shaft  326  (see FIG.  24 ). 
     The sprockets  328  and hub  426  are rearwardly biased along a rear end portion of the shaft  326 , toward a stop position created by the pin  428 , by a helical compression spring structure  432  that circumscribes the drive shaft  326  and bears at its opposite ends against the forward sprocket  328  and a rearwardly facing annular exterior shoulder surface  434  on the shaft. For illustrative clarity the spring structure  432  has been shown in FIG. 46 as being a single spring. However, the spring structure is preferably a nested, counterwound pair of helical compression spring members. 
     A central portion  332   a  of the link  332  on the ready cartridge  330   b is closely received between the facing side surfaces of the sprockets  328 . The sprockets  328  holds the link portion  332   a  and prevents it from moving forward. When the ready cartridge  330   b  is pushed forward by the bolt, the sprockets  328 , and their associated support structure  371 , are free to move forward a limited amount while still supporting the link  332  and ready cartridge  33   b  in the feeder. Thus, the forwardly moving cartridge  330   b  moves the link  332  forward which, in turn, forwardly moves the sprocket structure against the resilient resistance of the spring structure, the forward movement of the cartridge  330   b  being limited, as previously described, by the ogive bumper  372 . Additionally, when the bolt forwardly contacts the sprocket support structure face  370  the sprocket structure is forwardly moved against the resistance of the spring structure  432 . 
     The receiver  48  (see FIG. 5) has a top rear portion  48   a , and a pair of opposite left and right side plate portions  48   b  to which the previously mentioned recoil housings  58  are removably attached. The right receiver side plate  48   b  is shown in FIG. 47 with its recoil housing  58  (see FIG. 5) having been removed to uncover a resilient recoil assembly  436  secured to the outer side of the right receiver side plate portion  48   b . The left receiver side plate portion  48   b  (not shown in FIG. 47) has an identical resilient recoil assembly secured thereto and covered by its recoil housing  58 . In a manner subsequently described herein, the recoil assemblies  436  mounted on opposite sides of the gun  34  serve to resiliently and yieldingly resist the rearward firing recoil of the gun, and to create the necessary forward counter recoil movement of the fired gun. 
     FIG. 47 shows the right side recoil assembly  436  in the position it assumes when the gun  34  is stationary relative to its associated cradle structure  36 . With additional reference now to FIG. 48, the illustrated right resilient recoil assembly  346  includes, beginning from the rear end of the illustrated receiver side plate  48   a  and moving toward its front end, (1) a spring-loaded hydraulic recoil shock absorber  438  fixedly secured to a rear end portion of the side plate  48   b ; (2) a recoil block unit  440  which is not secured to the side plate  48   b ; (3) an elongated guide rod structure  442  circumscribed by a longitudinally spaced plurality of tubular recoil springs  444  (representatively four in number) and slidably extending through a plurality of tubular spring spacer members  445 ; (4) a recoil spring stop  446  fixedly secured to the side plate  48   b ; and (5) a hydraulic recoil damper  448  fixedly secured to the side plate  48   b.    
     Shock absorber  438  has a front end plunger portion  450  that extends toward an elastomeric bumper member  452  secured to the rear side of the recoil block  440 . The vertical pin  134  (see FIGS.  5  and  6 ), which is anchored to the cradle structure  36 , extends downwardly through an opening  454  in the recoil block  440 , thereby locking the block  440  to the cradle structure  36  relative to which the gun  34  is movable. 
     Springs  444  bear at their opposite ends against outwardly projecting flange portions  456  of the spacer members  445 , with a front longitudinal portion of the guide rod structure  442  slidably extending through an opening  458  in the recoil spring stop  446  and having a threaded front end  460 . Threaded front end  460  is secured to the outer end of a rearwardly extending plunger portion  462  of the recoil damper  448 . 
     As shown in FIG. 7, with the gun  34  at rest in its associated cradle  36  the recoil shock absorber plunger  450  (see FIG. 48) is fully depressed and forwardly engaging the elastomeric bumper  452 , the springs  444  are in partially compressed states, and the recoil damper plunger  462  is rearwardly extended. When the gun  34  is fired and placed in rearward recoil motion relative to its cradle  36 , the right receiver side plate  48   b  (like the left receiver side plate) is driven rearwardly relative to the stationary recoil block  44  as indicated by the arrow  464  in FIG. 49, and as may be seen by comparing FIGS. 47 and 49. 
     This rearward recoil movement of the receiver side plate  48   b  also moves the recoil shock absorber  438 , the recoil spring stop  446  and the recoil damper  448  rearwardly relative to the stationary recoil block  440 . In turn, as indicated in FIG. 49, this further compresses the springs  444 , which resiliently resist the gun&#39;s rearward recoil motion, moves the shock absorber plunger  450  away from the elastomeric bumper  452 , and forces the damper plunger  462  further into the body of the recoil damper  448  to thereby damp the gun&#39;s rearward recoil force. 
     When the gun reaches the rearward limit of its recoil travel, as previously discussed herein, the compressed springs  444  drive the gun forwardly through its counter recoil motion until the illustrated receiver side plate  48   b  is returned to its FIG. 47 position relative to the stationary recoil block  440 . As the gun reaches such FIG. 47 position, the shock. absorber plunger  450  contacts the elastomeric bumper  452  and is driven back into the body of the shock absorber  438 , the damper plunger  462  is extended further outwardly from the recoil damper  448 , and the shock absorber  438  is stopped against the stationary elastomeric bumper  452  which serves to cushion the shock of the shock absorber  438  being stopped by the stationary recoil block  440 . The damper  448  at the forward end of the assembly  436  serves to compensate for the possible adverse effects of spring frictional forces on the recoil absorbing capabilities of the recoil assembly  436  during recoil and counter recoil movements of the gun  34  relative to its support cradle  36 . 
     Turning now to FIGS. 50-55, the charger and sear assembly housing  50  mounted on the underside of the gun  34  is of a generally rectangular configuration (see FIGS. 53 and 55) and has a front end  466 , a rear end  468  from which a charger motor  470  rearwardly projects, top and bottom side walls  472  and  474 , and left and right side walls  476  and  478 . The left and right operating rods  266   a , 266   b  respectively extend through left and right portions of the housing  50 , passing through circular openings  480  in the front and rear ends  466 , 468  of the housing  50 . 
     The sear assembly  270  is carried in a front end portion of the housing  50  and, as previously mentioned, is used to prevent chambering of ammunition rounds prior to firing the gun  34 . As will now be described, the sear assembly prevents this round chambering by releasably holding the operating rods  266   a , 266   b  in their rearwardly shifted positions against the forward shifting forces exerted thereon by the drive springs  342  (see FIG.  17 ). To facilitate this releasable sear assembly holding of the operating rods  266   a , 266   b  in their rearwardly shifted positions, the top side of each of the operating rod front end caps  344  (see FIG. 52) has formed thereon a forwardly facing arcuate ledge  482 . Each ledge  482  has, as viewed from the front, a generally concave curvature. 
     Still referring to FIGS. 50-55, the sear assembly  270  includes (1) a vertically movable solenoid core  484  that carries the solenoid plunger  358  and is upwardly biased within the sear solenoid  356  by the coil spring  360 ; (2) left and right sear members  486  having rearwardly facing operating rod-engaging rollers  488  thereon; (3) a rotatable drive rod  490 ; (4) a sear link arm  492  positioned adjacent the right sear member  486 ; (5) a mechanically advantaged sear bar linkage having pivotally interconnected first and second bar portions  494 , 496  positioned adjacent the sear link arm  494 ; and (6) a drive ring  498 . 
     The sear members  486  are rotatably carried in well areas  500  on the top side  472  of the housing  50 , and are locked to the shaft  490  for conjoint rotation therewith. A rear end portion of the sear link arm  492  (see FIG. 50) is also locked to the shaft  490 , with a front end of the sear link arm  492  underlying a rearwardly projecting front end portion  496   a  of the sear bar  496 . The rear end of the sear bar  494  is pivotally connected to the rod  490 , the rear end of the sear bar  496  is pivotally connected to a longitudinally intermediate portion of the sear bar  494 , the front end of the sear bar  494  is pivotally connected to a drive pin  502  carried on the top end of the solenoid core  484 , and the front end of the sear bar  496  is pivotally connected to an adjacent portion of the housing  50 . 
     The drive ring  498  (see FIG. 54) circumscribes the drive rod  490 , is rotatably carried on an inner side portion  500   a  of the left sear well  500 , and is rotationally locked to the drive rod  490 . A generally tangential notch  504  is formed in the bottom side of the drive ring  498  and receives the front end of a biasing plunger  506  (see FIGS. 54 and 55) which is forwardly driven by a coil spring  508  (see FIGS.  53  and  55 ). Via the action of the spring-driven plunger  506  on the drive ring  498  the drive ring  498  (and thus the drive rod  499 ) is rotationally biased in the direction of arrow  510  in FIG.  54 . 
     This, in turn, rotationally biases the left and right sear members  486  rearwardly and downwardly to their “on sear” positions (with the solenoid  356  de-energized) in which their rollers  488  (see FIG. 50) engage the operating rod front end cap arcuate ledges  482  in a manner releasably holding the operating rods  266   a , 266   b  in their rearwardly retracted “on sear” positions. With the operating rods  266   a , 266   b  in these positions the components of the sear assembly  270  are in their FIG. 50 orientations, with the solenoid core  484  being in its “up” position. 
     When the gun  34  is fired, electrical current is sent to the sear solenoid  356  and causes the core  484  to be driven downwardly, against the resistance of the spring  360 , to its FIG. 51 position. This, in turn, forcibly pivots the sear bar linkage  494 , 496  in a manner causing it to exert a mechanically advantaged pivotal force on the sear link arm  492  to cause it to upwardly pivot the sear rollers  488  part way up the arcuate end cap ledges  482 . At this point the rollers  488  are moved upwardly past a “pinch” angle on the ledges  482  to permit the forwardly spring-biased ledges  482  (via the operating rod drive springs  342 ) to drive the rollers further upwardly, thereby permitting the ledges  482  to be forwardly driven past the sear members  488  (see FIG. 52) and freeing the operating rods  266   a , 266   b  from the sear assembly  270 . 
     During firing of the gun  34  after this release of the sears  486  from the operating rods  266   a and  266   b , the downwardly displaced core  484  of the still energized solenoid  356  rotationally displaces the sear members  486  upwardly (against the opposite downward sear biasing force of the drive ring plunger spring  508 ) a sufficient distance such their rollers  488  are above the pinch angle of the end cap ledges  482 . Accordingly, every time during a firing burst that the ledges  482  pass the sear rollers  488  they simply upwardly displace them without being stopped thereby. 
     When the solenoid  356  is subsequently de-energized, the resulting upward movement of its core  484  causes the sear rollers  488  to be rotationally lowered to their FIG. 50 locking positions, as the operating rod end cap ledges  482  rearwardly pass them, to again lock the operating rods in their rearwardly retracted “on sear” positions. 
     It should be noted that, in sharp contrast to conventional sear design, the sears  486  need not be pulled completely out of the path of their associated operating rod stop surfaces to effect the locking of the operating rods  266   a , 266   b  in their rearwardly retracted positions. Additionally, because of the mechanically advantaged sear release linkage, and the use of the arcuate shape of the end cap ledges  482  to partially raise the sear rollers  488 , the required size of the solenoid  356  is substantially reduced. 
     Turning now to FIGS. 56-61, during certain pre-firing conditions of the gun  34  (such as when has just been reloaded), the operating rods  266   a , 266   b  are in their forwardmost positions within the gun and must be retracted to their “on sear” positions (see FIG. 50) before the gun can be fired. This initial retraction of the operating rods  266   a , 266   b  is performed in the gun  34  using a specially designed charger assembly  512  which is carried in a rear portion of the housing  50  rearwardly of the previously described sear assembly  270 . 
     The previously mentioned drive motor portion  470  of the charger assembly  512  is positioned on the rear end of the housing  50  and has a forwardly extending drive shaft  514  connected to a bevel gear  516  which is meshed with an intermediate gear  518  having a stem pinion secondary gear portion  520  thereon. Stem pinion gear portion  520 , in turn, meshes with a pinion drive gear  522  having an internally grooved hub  524  (see FIG. 61 that slidably receives a splined end portion  526  of a pinion gear  528 . Pinion gear  528  (see FIG. 57) is slidably received in a central opening in a ball bearing  530  supported on the right side wall  478  of the housing  50 . As can be seen by comparing FIGS. 57 and 58, the pinion gear structure  526 , 528  is rotationally locked to the pinion drive gear  522 , but can slide in left and right directions relative thereto. 
     The splined end portion  526  of the pinion gear  528  forms the core of a charger solenoid  532  and is biased rightwardly within the solenoid housing  534  by a coil spring  536  which circumscribes a leftwardly extending plunger  538  secured to the splined end portion  526 . Plunger  538  is slidably extended through a suitable support portion  540  within the housing  50 , and the spring  536  bears at its opposite ends against the left end of the splined end portion  526  and the support portion  540 . 
     With the charger solenoid  532  de-energized, the pinion gear structure  526 , 528  is spring-driven rightwardly to its position shown in FIGS. 57 and 61. In this position of the pinion gear structure  526 , 528  a nontoothed central portion  542  thereof (see FIG. 57) underlies the right operating rod  266   b  (not shown in FIG. 57) and does not drivingly engage it. Thus, with the charger solenoid  532  de-energized the charger assembly  512  does not interfere with the forward and rearward movement of the right operating rod  266   b.    
     When it is necessary to charge the gun  34 , (with the operating rods in their forwardmost positions within the gun) the charger solenoid  532  is energized to thereby leftwardly shift the splined end portion  526  of the pinion gear to its FIG. 58 position (as indicated by the arrow  544  in FIG. 58) against the force of the spring  536 , thus also moving the plunger  538  to its FIG. 61 dotted line position. This leftward shift of the pinion gear  528  brings its teeth into meshed engagement with a gear rack  546  formed on a flattened underside portion  548  of the right operating rod  266   b  (see FIGS.  59  and  60 ). 
     Operation of the charger motor  470  then rotationally drives the pinion gear  528  to rearwardly drive the right operating rod  266   b  (and thus the left operating rod  266   a ), as indicated by the arrow  550  in FIG. 60, to its FIG. 50 “on sear” position at which point the charger motor  470  and solenoid  532  are de-energized. The de-energization of the solenoid  532  rightwardly shifts the pinion gear structure  526 , 528  back to its FIG. 57 position to disengage it from the right operating rod  266   b  and ready the gun  34  for firing. 
     Conventional gun charging structures typically utilize a charging drive member which must move rearwardly along the gun to force the operating structure to its on sear position, and then be moved forwardly away from the now locked operating structure to be out of its way when the gun is subsequently fired. As is well known, this tends to introduce a substantial amount of additional complexity into an already complex mechanical apparatus. In the present invention, however, all parts of the charger assembly  512  remain stationary in a front-to-rear direction relative to the gun during the charging operation, and are transversely shifted only a short distance to effect rapid and simple disconnection of the charger assembly from the operating rods. 
     As can be seen in FIGS. 57-59, the outer stem end  552  of the intermediate gear  518  has a hex head configuration and is recessed within an access opening  554  in the right side wall  478  of the housing  50 . This permits the intermediate gear  518  (and thus the pinion gear  528 ) to be manually rotated to the charge the gun  34  if, for example, the charger motor  470  is inoperative. 
     With reference now to FIG. 62, a small brush housing  50   a  is mounted on the charger/sear housing  50  on a left rear portion thereof (see FIGS. 53 and 55) and has an interior portion  556  through which the left operating rod  266   a  longitudinally extends. Pivotally mounted within the interior of the brush housing  50   a  are two electrical contact brushes—a primer power brush  558 , and a bolt unit aft brush  560 . Brushes  558 , 560  are spring-loaded in a manner such that they are pivotally biased, as indicated by the arrows  562 , 564  in FIG. 62, into sliding engagement with later described outer side surface portions of the left operating rod  266   a  as it is longitudinally cycled back and forth through the brush housing  50   a  as indicated by the double-ended arrow  566  in FIG.  62 . Electrical leads, schematically shown by the dotted lines  568 , 570  in FIG. 62, are respectively connected to the brushes  558  and  560 . 
     As shown in FIG. 70, a first elongated strip  572  of electrically insulative material longitudinally extends along the outer side surface of the left operating rod  266   a , on a longitudinally intermediate portion thereof, and has front and rear ends  572   a  and  572   b . A second elongated layer  574  of electrically insulative material is circumferentially spaced apart from strip  572  and longitudinally extends along a rear portion of the outer side surface of the left operating rod  266   a , the layer  574  having a front end  574   a . Exposed on the forward end of the insulative layer  574  is an electrically conductive hot shoe  576  (see FIG.  63 ). An electrical wire  578  (see FIGS. 63 and 70) is imbedded in the insulative layer  574  and electrically connects the hot shoe  578  and the operating rod body structure  284 . 
     Low voltage DC electrical power from the gun&#39;s electrical system is continuously routed via lead  570  (see FIG. 62) to the bolt unit aft brush  560  which longitudinally slides along the insulative layer  572  during operation of the gun. As the left operating rod  266   a  reaches its rearwardmost position within the gun, the brush  560  moves off the front end  57   a  of the insulative layer  572  and is grounded to the adjacent bare metal portion of the left operating rod  266   a . This grounding creates a signal which may be used by the gun&#39;s electrical system to monitor the front-to-rear position of the operating rod structure  266 , 284  relative to the balance of the gun. Additionally, during charging of the gun  34  this sensed grounding of the brush  560 , which indicates that the operating rods have been moved rearwardly to their “on sear” positions, is used to cause the electrical system to turn off the charger motor  470 . 
     Turning now to the electrical firing circuit diagram in FIG. 63, a suitable control box  580  receives 28V DC electrical power through lead  582  and outputs the power via lead  584  to a normally open double pole trigger switch  586  which, when closed, routes 28V DC electrical power to the sear solenoid  356  through lead  588 , and to a bolt face sensor circuit  590  through lead  592 . When the bolt sensor switch  368  (see FIGS. 43 and 46) is closed as previously described, it transmits an electrical signal  594  to the bolt face sensor circuit  590  which responsively transmits 28V DC current via lead  596  to a high voltage conversion circuit  598 . 
     In response to the receipt of 28V DC electrical power through lead  596 , the circuit  598  outputs 300V high frequency pulsed DC electrical power to the brush  558 . The brush  558 , in turn, conducts this high voltage electrical power to the hot shoe  576  on the left operating rod  266   a , the hot shoe  576  being aligned with the brush  558  only when the bolt unit  260  is in its forward position and closing the bolt sensor switch  368 . High voltage firing current received by the hot shoe  576  is passed through its associated electrical lead  578  to the operating rod body structure  284  (see FIG.  70 ). In turn, as illustrated in FIG. 27, this firing current is sequentially passed through the electrical probe  376 , the operating rod body hot shoe  378 , and to the electrical firing contact structure  382  of the gun. 
     Thus, the primer power brush  558  is electrically energized only when the gun&#39;s bolt structure is forward and closed, and the gun&#39;s electrical firing power supply is intermittent. This provides a significant advantage over the conventional firing power supply technique of continuously maintaining electrical firing power and utilizing such power when the firing components of the gun reach a predetermined alignment. In the present invention, such electrical firing power is generated only if such components are in their firing orientations. 
     One of the many advantageous design features of the gun  34  is its ease of field serviceability. This field serviceability improvement includes a unique barrel lock system which permits a twist lock and unlock installation and removal of the barrel  52  as will now be described in conjunction with FIGS. 64-69. 
     As illustrated in cut away perspective form in FIG. 67, a rear end portion of the barrel  52  has an axially spaced series of radially outwardly projecting arcuate lugs  600  on an outer side surface thereof. To install the barrel  52  within the gun, the barrel is rearwardly inserted, rear end first, into the front end of the gun (through a circular passage  602  in the front end unit  348  as can be best seen in FIG. 68) with the lugs  600  facing down as shown in FIG.  67 . When a rear end portion of the inserted barrel enters a tubular passage  604  within a barrel receiving block member  606  (see FIG. 69) within the gun, the barrel  52  is twisted 90 degrees in either direction to interlock its lugs  600  with corresponding sets of spaced lugs  608  disposed on opposite left and right interior side surface portions in within the block member  606  (only the left set of lugs  608  being visible in FIG.  69 ). This lug interlock prevents the inserted longitudinal portion of the barrel  52  from moving forwardly out of the gun body until the barrel is twisted 90 degrees in either direction relative to the gun body. 
     The interlockable lugs  600 , 608  form a portion of a specially designed barrel lock system  610  of the present invention, the balance of the lock system  610  being carried on the front end unit  348  as shown in FIGS. 64-68. In addition to the lugs  600 , 608  the lock system  610  also includes a diametrically opposite pair of lateral exterior recesses  612  formed in the gun  52 ; a vertically oriented barrel lock pin  614  with diametrically opposite flat sides  616  on its upper end and a vertical slot  618  passing through its lower end; a horizontal retaining pin  620 ; and a generally disc-shaped bottom handle  622  which is axially locked to the bottom end of the barrel lock pin  614  but is rotatable relative thereto. 
     Referring now to FIGS. 64 and 68, the function of the barrel lock pin  614  is to releasably prevent the inserted barrel  52  from being twisted from being rotated in a manner disengaging its lugs  600  from the associated gun lugs  608 . This prevention of barrel unlocking rotation is achieved by the receipt of the upper end of the barrel lock pin in one of the barrel recesses  612  (see FIG.  64 ), the flat sides  616  of the upper end of pin  614  engaging corresponding flat sides of the barrel recess  612  that receives the upper lock pin end. 
     The lock pin  614  is received in a vertical passage  624  formed in the body of the front end unit  348  and having an open bottom end. The retaining pin  620  extends through the body of the front end unit, and through the lock pin slot  618 , and retains the lock pin  614  in the vertical passage  614  for limited vertical movement therein between an upper limit position (see FIG. 64) in which the upper end of the lock pin  614  may be received in one of the barrel recesses  612  to prevent the barrel  52  from rotating relative to the balance or the gun, and a lower limit position (see FIG. 65) in which the upper end of the lock pin  614  is withdrawn from the barrel recess  612  to permit the barrel  52  to be twisted to decouple the lugs  600 , 608  and permit the barrel  52  to be pulled out of the gun. 
     The handle  622  has, on its upper side, a central upstanding cylindrical boss  626  having an outwardly projecting annular flange  628  on its top end which forms a circular groove  630  between the flange and the top side of the handle  622 . The flange  628  has a diametrically opposite pair of flat areas  632  formed thereon. When the lock pin  614  is in its FIG. 64 barrel locking position, the retaining pin  620  is at the lower end of the locking pin slot  618 , and the bottom handle is turned to its FIG. 64 position in which holding pins  634  (see also FIG. 68) tangentially extend through diametrically opposite portions of the groove  630  beneath the nonflattened portions of the boss flange  628  to thereby prevent the handle from being moved downwardly away from the body of the front end unit  348  and thereby permitting the upper end of the lock pin  614  to be downwardly moved out of the barrel recess  612  that receives it. A detent structure  636  carried by the front end unit  348  has a spring-loaded detent ball portion  638  on its lower end which is received in one of a circumferentially spaced plurality of detent recesses  640  formed on the top side of the handle  622  to resiliently prevent it from being rotated relative to the front end unit body and permit lowering of the lock pin  614 . 
     A gas system plug  642  is threaded upwardly into the open lower end of a cylindrical passage  644  in the front end unit body and has a circular lower end handle  646  with a circumferentially spaced series of notches  648  therein. A vertical detent structure  650  is carried by the front end unit  348  and has on its lower end a spring-loaded detent member  652  that is received in one of the handle notches  648  to resiliently prevent loosening rotation of the handle  646 . 
     To remove the barrel  52  from the gun  34 , the lock pin handle  622  is rotated from its FIG. 64 position to its FIG. 65 position to rotate the nonflattened portions of the flange  628  away from their overlying relationships with the holding pins  634 , and then pulling the handle  622  down to its FIG. 65 position in which the pin  620  hits the upper end of the lock pin slot  618  and the upper end of the lock pin  614  is withdrawn from its associated barrel recess  612 , thereby rotationally unlocking the barrel  52  and permitting it to be rotated relative to the balance of the gun. Next, as sequentially illustrated in FIGS. 66 and 67, the barrel  52  is rotated 90 degrees (see FIG. 66) to unlock its lugs  60  from the associated gun lugs  68 , and then pulled forwardly out of the gun as shown in FIG.  67 . 
     To reinstall the barrel  52  in the gun  34  the process is simply reversed. Specifically, the barrel  52  is inserted into the gun, rotated 90 degrees to interlock the lugs  60  and  68 , the lock pin  614  is pushed upwardly to its FIG. 64 position in which the top end of the lock pin  614  enters one of the barrel recesses  612 , and the lock pin handle  622  is rotated to its FIG. 64 position in which it is releasably locked to the body of the front end unit  348 . 
     In addition to this field serviceability feature of the gun  34 , the entire operating rod structure, and associated bolt buffer structure and rod guiding structure may be quickly removed from the gun for cleaning, inspection and maintenance purposes simply by removing two retaining ball pins  654  from the gun  34  (see FIGS. 5 and 70) from the rear end of the gun  70 . These retaining pins  654  extend through circular side portion holes  656 , 658  in the rear end of the receiver portion  48   a  (see FIG. 70) and underlying holes  656   a , 658   a  in rear end portions of the opposite receiver side plates  48   b . The simple and rapid removal of the pins  654 , as shown in FIG. 70, permits the receiver portion  48   a , the guide rods  338  and bolt buffer rods  398  secured thereto, the operating rod structure  284 , 366  and the bolt assembly  260  to simply be pulled rearwardly away from the balance of the gun  34  without requiring the use of any disassembly tools. Compared to machine guns of conventional construction, this greatly simplifies the access to the internal components of the gun. This serviceability is further enhanced by the formation, as shown in FIG. 69, of the front receiver portion of the gun in two separable halves  348   b  which are held together by easily removable screws  660 . 
     Another desirable feature of the gun  34  is its substantially reduced firing recoil force compared to 30 mm machine guns of conventional construction. As previously mentioned, this sharp reduction in firing recoil force stems partly from the ability of the gun to be fired in an “out of battery” condition—i.e., while the gun is still moving forwardly in its associated cradle structure during counter recoil movement of the gun subsequent to a previous shot. 
     Additionally, the entire charger assembly is formed as an integral part of the gun for movement with the gun during firing thereof. This is in sharp contrast to the conventional practice of constructing and positioning the charger in a manner such that it does not move with the gun during firing thereof. Because of this construction the charger does not form a portion of the gun&#39;s recoil mass, and is thus merely “parasitic” weight. The present invention&#39;s mounting of the charger directly on the gun, so that the charger becomes a part of the gun&#39;s recoil mass, advantageously reduces the resulting recoil force of the gun. 
     Another unique feature in the gun  34  that desirably reduces its recoil force is a special construction incorporated by the present invention into the previously mentioned muzzle brake  54  shown in FIGS.  5  and  15 - 17 . Specifically, the muzzle brake  54  has an elongated hollow body, with open front and rear ends  662 , 664 . The rear muzzle brake end  664  is threadable onto the outer end of the barrel  52  so that the longitudinal axis of the internal passage  666  of the muzzle brake  54  is coaxial with the firing axis of the barrel  52 . 
     Extending outwardly from opposite top and bottom sides of the muzzle brake internal passage  666  through corresponding top and bottom exterior side surfaces of the muzzle brake body are an axially spaced series of gas discharge openings  668  which are angled rearwardly and laterally outwardly, preferably at angles of approximately thirty degrees, relative to a plane transverse to the longitudinal axis of the muzzle brake. As can be best seen in FIG. 5, the cross-sectional areas gas discharge openings  668  progressively increase in a rear-to-front direction along the length of the muzzle brake  54 . The rearward slope of the gas discharge openings  668  causes pressurized gas outwardly discharged therefrom impart a forward force on the gun, offsetting part of the gun&#39;s rearward recoil force subsequent to firing thereof, while the unique progressively increasing cross-sectional areas of the opening  668  tends to even out this forward force during its existence by providing more gas outlet area as the internal gas pressure in the muzzle brake decreases with each successive uncovering of a more forward opening  668  by a cartridge projectile being gas-propelled through the interior of the muzzle brake. 
     Another unique aspect of the present invention which desirably reduces the firing recoil force of the gun  34  arises from the substantial over pressurization of the previously mentioned cylinder areas  386  in the front end unit  348  (see FIGS. 30 and 68) beyond the minimum pressure level required therein to rearwardly drive the operating rod structure. 
     As can be seen from the foregoing, the present invention provides a 30 mm machine gun that, compared to its conventional counterparts, provides a less complex construction, substantially improves the ability to field service the gun, and markedly reduces the gun&#39;s firing recoil forces to thereby correspondingly improve the gun&#39;s controllability and firing accuracy. While the 30 mm machine gun of the present invention has been representatively illustrated and described as being cradle-mounted on an end of an aircraft support plank, it will be readily appreciated by those of skill in this particular art that it could be operatively supported on a variety of other aircraft and ground support structures if desired. Additionally, while the principles of the present invention have been illustratively incorporated in a 30 mm machine gun, it will also be appreciated that such principles could also be incorporated in a variety of other types and sizes of machine guns if desired. 
     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.