Abstract:
Disclosed is an upper receiver assembly for use with an AR15-type lower receiver. The upper receiver assembly includes a monolithic upper receiver and forearm housing. A barrel has a breech end, a muzzle end, a bore axis, and a barrel trunnion at the breech end having a lug for direct connection to the lower receiver using the pivot pin. It includes a bolt carrier assembly, having a longitudinal slot and rail engagement with the upper receiver housing on which the bolt carrier assembly slidably reciprocates, and a long-stroke gas-piston actuation system. A recoil spring assembly is housed within the monolithic upper receiver and forearm housing and extends coaxially with the longitudinal axis of the gas-piston actuation assembly. The barrel trunnion and gas block are configured for engagement with the upper receiver and forearm housing with longitudinal keyway features.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/737,947 filed Dec. 17, 2012. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention generally relates to a gas piston unitary upper receiver system for an automatic or semiautomatic firearm, particularly for use with a standard AR15/M16 lower receiver. 
       BACKGROUND OF THE INVENTION 
       [0003]    Issues with the direct impingement operation system of the AR15/M16 rifle (and variants thereof) are well known. Many attempts have been made to replace the original direct impingement operation system with a gas piston system. Some proposals are retrofit systems, in which the original gas tube is replaced with a piston and cylinder for short stroke operation of the bolt. These systems typically use the existing buffer spring that extends into the butt stock. Other solutions have been proposed in which an entirely new rifle is designed to include operator controls similar to those familiar in the AR15/M16 platform. For example, Remington Arms has produced its Adaptive Combat Rifle (ACR) and FN Herstal (FNH USA) has proposed its MK16 and MK17 rifles, the latter of which was adopted by the United States Special Operations Command (SOCOM) as a result of the Special Operations Forces Combat Assault Rifle (SCAR) competition. Each of these examples includes a recoil spring within the upper receiver, allowing the butt stock to be folded, if desired. These designs, however, utilize a unique (nonstandard) lower receiver and require complete replacement of existing rifle systems, rather than allowing modification of existing weapons currently in inventory. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a number of features. It has a unitary upper receiver that mounts to any standard AR15/M16 lower receiver. The upper receiver, which houses a reciprocating bolt carrier assembly is unitary with a forearm portion which supports a barrel assembly, gas piston operation system, and charging handle. The gas operation system includes a long-stroke piston for positive operation of the bolt carrier assembly without introduction of dirty gases into the upper receiver and bolt carrier area. This allows the use of a non-reciprocating charging handle, which is fully ambidextrous and reduces the mass of reciprocating parts. 
         [0005]    Unique geometry of the bolt carrier allows it to reciprocate on replaceable rails, which reduces the frictional contact area, can be provided with self-lubricating coatings, and allows axial alignment between the operation rod and recoil spring. According to one embodiment, a novel bolt is provided which allows it to be repositioned by the user for left- or right-handed (fully ambidextrous) operation. The unique geometry of the recoil spring and operation rod allows recoil forces of the reciprocating bolt carrier and piston rod assembly to be transferred to a robust portion of the upper receiver, rather than through a recoil spring and buffer housed in the butt stock of a typical AR15/M16 rifle platform, without tipping the bolt carrier. 
         [0006]    The invention also provides a quick-change barrel assembly, held in place by a lower forearm cover, with a barrel trunnion that connects directly to the pivot pin of the lower receiver. The barrel assembly, comprising a barrel, barrel trunnion, gas block, gas regulator valve, and gas cylinder, can be easily removed and replaced as a unit. This allows quick change in barrel length, caliber, and twist rate, as desired by the user. Because the gas regulator valve and gas cylinder are part of the interchangeable barrel assembly, the gas control system may be matched to the characteristics of the barrel (caliber, and length, and twist rate) without any change to the gas piston, which remains connected to the bolt carrier assembly. 
         [0007]    Because the recoil spring and buffer system is housed within the upper, it may be used with a folding stock in a rifle system or without any rearward protrusion in a pistol platform. 
         [0008]    According to another feature of the invention, a clean-out port is provided in the gas block in axial alignment with the gas port and gas passageway in the barrel and gas block. When the gas regulator valve is removed, this allows direct access for cleaning these passageways without any other disassembly of the unit. 
         [0009]    According to another feature of the invention, a two-stage recoil spring and buffer system may be used. This system provides rapid deceleration of the bolt carrier assembly during the final portion of rearward cycle. 
         [0010]    According to another feature of the invention, a bolt having improved curvilinear lug geometry provides increased strength and resistance to cracking by minimizing or eliminating sharp edge cuts in the bolt face profile. 
         [0011]    Other features, benefits, and combinations will be apparent from the various figures of the drawing and detailed description of preferred embodiments herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Like reference numerals are used to indicate like parts throughout the various figures of the drawings, wherein: 
           [0013]      FIG. 1  is a right side plan view of an upper assembly according to an embodiment of the invention shown mounted on a stripped lower receiver; 
           [0014]      FIG. 2  is a view similar to  FIG. 1 , showing the upper receiver body, lower receiver, and lower forearm cover cut-away in longitudinal cross-section; 
           [0015]      FIG. 3  is an isometric rear-side view of the upper receiver assembly shown in  FIG. 1 ; 
           [0016]      FIG. 4  is a pictorial view similar to  FIG. 3  in which the upper receiver body has been removed to expose the inner working parts of the device; 
           [0017]      FIG. 5  is a fragmentary longitudinal sectional view of the gas block and gas regulator valve; 
           [0018]      FIG. 6  is a cross-sectional view taken substantially along line  6 - 6  of  FIG. 5 ; 
           [0019]      FIG. 7  is a fragmentary pictorial view of the bolt carrier assembly; 
           [0020]      FIG. 8  is a longitudinal sectional view taken substantially along line  8 - 8  of  FIG. 7 ; 
           [0021]      FIG. 9  is a view similar to  FIG. 8  showing the firing pin retention gate lifted and the firing pin partially removed; 
           [0022]      FIG. 10  is an exploded pictorial view of the bolt carrier assembly showing how the bolt is repositioned to change between left-handed and right-handed operation; 
           [0023]      FIG. 11  is a plan view of the face of a prior art bolt; 
           [0024]      FIG. 12  is an end plan view of a bolt face according to one embodiment of the present invention; 
           [0025]      FIG. 13  is a cross-sectional view taken substantially along line  13 - 13  of  FIG. 14 , but with the bolt carrier assembly shown in a retracted position; 
           [0026]      FIG. 14  is a fragmentary pictorial view of a rear portion of the upper receiver assembly; 
           [0027]      FIG. 15  is a fragmentary pictorial view of the other side thereof; 
           [0028]      FIG. 16  is a partially exploded view similar to  FIG. 14 ; 
           [0029]      FIG. 17  is a partially exploded view similar to  FIG. 15 , but with the interchangeable parts reversed for left-handed operation; 
           [0030]      FIG. 18  is a fragmentary, partially cut-away top plan view of a forward portion of the charging handle body showing positions of the charging handle lever; 
           [0031]      FIGS. 19-22  are a series of fragmentary partially cut-away views of a rear portion of the upper receiver assembly illustrating the steps involved in field stripping disassembly; 
           [0032]      FIG. 23  is a fragmentary partially cut-away view of a forward portion of the receiver body and barrel assembly; 
           [0033]      FIG. 24  is a disassembled view thereof; 
           [0034]      FIG. 25  is a pictorial exploded view of the upper receiver body, barrel assembly, and lower forearm cover; 
           [0035]      FIG. 26  is a view of the upper assembly showing a two-stage recoil spring system in a first, in battery position; 
           [0036]      FIG. 27  is a view Like  FIG. 26  showing the two-stage recoil spring system in a second, less-than-fully-retracted position; 
           [0037]      FIG. 28  is a view Like  FIGS. 26 and 27  showing the two-stage recoil spring system in a third, fully retracted position; 
           [0038]      FIG. 29  is a graph illustrating the force curve of the two-stage recoil spring; 
           [0039]      FIGS. 30 and 31  are isometric views of a bolt having a novel lug profile; 
           [0040]      FIG. 32  is a longitudinal sectional view taken substantially along line  32 - 32  of  FIG. 33 ; and 
           [0041]      FIG. 33  is an enlarged end plan view of the bolt face profile. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    Referring to the various figures of the drawing, and first to  FIG. 1 , therein is shown a firearm having a gas piston operated unitary upper receiver assembly  12  mounted on a standard AR15/M16 lower receiver assembly. The lower receiver  14  may be that of a standard AR15/M16 rifle (or pistol). As used herein, the terms AR15, M16, M4, and other variants of these firearm platforms are used interchangeably. The present invention is operable in semiautomatic mode or may be operable in full automatic mode with appropriate modification.  FIGS. 1-4  illustrate a lower receiver  14  without a fire control group (lower parts kit), butt stock, hand grip or magazine. A butt stock mountable to any standard AR platform variant can be attached at  16 , a hand grip at  18 , and a removable magazine inserted into a magazine well  20 . A wide variety of stocks, grips, and accessories are available to interface with the standard AR15 lower receiver  14 . Specifics of these parts are not important to the present invention, and for sake of simplicity, are not shown in the other figures. The upper receiver assembly  12  of the present invention is fully usable with a lower receiver  14  in a pistol configuration (not shown). The present invention also allows use of a folding stock (not shown), if desired, or continued operation after the butt stock has been bent or broken. 
         [0043]    The upper receiver assembly  12  attaches to the lower receiver  14  using the standard takedown pin  22  and pivot pin  24 . By utilizing a standard AR15/M16 platform lower receiver  14 , existing weapons systems may be upgraded without replacement, particularly of the serialized lower receiver  14 , legally considered to be the “firearm.” 
         [0044]    The upper receiver assembly  12  includes a unitary upper receiver body  26  which may be billet milled or otherwise formed of a suitable material, such as an aluminum alloy. Alternatively, an extrusion of a suitable profile may also be formed and milled to final specifications or could be molded from a suitable polymer material. 
         [0045]    As shown in  FIGS. 1 and 3 , an ejection port  28  is provided on one side, in this example, the right-hand side. An elongated charging handle opening  30  is provided on each side of the receiver body  26  to allow longitudinal movement of a laterally extending charging handle lever  32 . The charging handle lever  32  may be switchable without tools from either side to the other at the operator&#39;s discretion. The upper receiver body  26  includes a unitary fore end (forearm) and may include an integral top Picatinny (MIL-STD 1913) accessory mounting rail  34 . Additional accessory rails  36 ,  38  may be provided at the 9 o&#39;clock and 3 o&#39;clock positions. A separate lower forearm cover  40  may also include a bottom accessory rail  42 .  FIGS. 3 and 4  show the upper assembly  12  separate from the lower receiver assembly. Unlike a typical AR15 variant receiver having a rear charging handle, the upper receiver assembly  12  of the present invention is completely closed at its rear end, which is adjacent the operator&#39;s face when shooting, precluding any undesired escape of gases or combustion particles and providing enhanced strength. 
         [0046]    Referring now to  FIGS. 2 ,  3  and  4 , therein can be seen a gas piston operating system according to the present invention.  FIG. 2  is a side plan view showing the upper receiver assembly  12  and a stripped lower receiver  14  in longitudinal section.  FIG. 4  shows an isometric view of the entire internal system, but with the upper receiver body  26  being shown cut away in cross-section. The trigger and hammer mechanism parts of the lower receiver  14  are not shown for sake of clarity and because they do not form any part of the present invention. The trigger assembly and selector switch for any standard semiautomatic or fully automatic AR variant will function with the present invention. 
         [0047]    Referring now in particular to  FIGS. 2 ,  4 ,  7 ,  19  and  20 , the present invention provides a novel bolt carrier assembly  48 . The bolt carrier assembly  48  has a body  144  that is relatively square in cross-section rather than the typical round shape used in a direct impingement or gas piston conversion AR15/M16 variants. Also, rather than having a relatively large surface area of the bolt carrier in sliding contact within a generally tubular upper receiver, the present bolt carrier body  49  has lateral guide channels  54 ,  56  which engage and slide along left and right guide rails  58 ,  60 . In preferred form, the guide rails  58 ,  60  are made of a hardened and wear-resistant material, such as steel, and may include a lubricious coating. These guide rails  58 ,  60  are fixed into channels formed longitudinally inside the upper receiver body  26  and may be secured by a series of threaded fasteners  62 . The reciprocation track provided by these guide rails  58 ,  60  is closely adjacent to and parallel with the axis of the gas piston system (to be described below) and is vertically situated between the gas-piston/recoil-spring axis and the bore axis of the barrel. This orientation produces less angular loading on the bolt carrier body  49  and other parts that reciprocate when the action is cycled. 
         [0048]    Operation of the gas piston system can be seen by reference to  FIGS. 2 , and  4 - 6 . The barrel  70  includes a gas port  72 . A gas block  74  is positioned on the barrel  70  and fastened such as by pinning. The gas block  74  includes a gas passageway  76  and houses an adjustable gas valve  78 . Referring now particularly to  FIG. 6 , the gas valve  78  may be rotatably adjusted to provide a larger or smaller orifice, or may be positioned to completely close the gas passageway  76  for operation in a single shot mode. Accordingly, adjustment of the gas valve  78  allows the gas system to be “tuned” as required by variations in ammunition or use of a noise suppressor. 
         [0049]    In autoloading mode, the gas valve  78  directs gas pressure from the bore of the barrel  70  into a gas tube or cylinder  80 , which is affixed to and extends rearwardly from the gas block  74 . A piston rod  82 , having a piston head  84 , is positioned within the cylinder  80  and is acted upon by gases ported from the barrel  70  through the gas block  74  and valve  78 . The piston rod  82  extends rearwardly to an integral operation rod (op rod)  86 , which is attached by threaded engagement to the forward bolt carrier lug  50  of the bolt carrier body  49 . 
         [0050]    Referring now also generally to  FIGS. 26-28 , when gas pressure bears upon the piston head  84 , the piston rod  82  and op rod  86  are shifted rearwardly in a long-stroke operation to cycle the bolt carrier assembly  48  against the force of the recoil spring  66 . The op rod  86  may be tubular, as shown, in order to reciprocate over the guide rod  68  and spring  66 . This tubular portion of the op rod  86  may also be skeletalized with holes in order to reduce weight. Unless the bolt carrier  49  is locked back in an open position, the recoil spring will return these parts to their forward, in-battery position ( FIGS. 2 ,  4  and  26 ). As is apparent from the figures, the gas piston  82 , op rod  86 , recoil spring  66 , and its guide rod  68  are all axially aligned. The bolt carrier assembly  48  reciprocates along a parallel track on closely adjacent guide rails  58 ,  60  which are situated between the bore axis of the barrel  70  (and bolt  44 ) and the axis of the piston  82 , op rod  86 , and recoil spring  66 . Thus, angular forces and any tendency for tipping of the bolt carrier assembly  48  are minimized. 
         [0051]    Referring to  FIGS. 5 and 6 , according to another feature of the invention, a cleaning port  96  may be provided in the gas block  74  in axial alignment with the gas passageway  76  and gas port  72 . When the adjustable gas valve  78  is removed, this allows cleaning access to the gas passageway  76  and gas port  72  without any other disassembly being required. 
         [0052]    The barrel assembly  110  includes the barrel  70 , barrel trunnion  98 , gas block  74 , gas pressure control valve  78 , and gas cylinder  80 . The bolt carrier assembly  48  includes a bolt carrier body  49 , a forwardly-extending tubular op rod  86 , gas piston  82 , bolt  44 , and firing pin  46 . The recoil buffer assembly  112  includes a removable buffer block  114 , a rear closure plate  116 , a guide rod  154 , and recoil spring  156 . 
         [0053]    The action may be manually cycled by pulling the charging handle lever  32  (shown in a forwardly folded position in  FIGS. 3 ,  4 , and  6 ) rearwardly to slide the charging handle body  88 . The charging handle body  88  includes forward and rearward laterally-extending guide ribs  92  which travel in longitudinal guide grooves  94  formed in the upper interior portion of the receiver body  26 . The charging handle body  80  also includes a downwardly-extending lug  90  which provides a one-way engagement with the forward end of the op rod  86 . In preferred form, the charging handle body  88  and charging handle lever  32  do not reciprocate as the gas piston system cycles, but may be used to manually cycle the action against the recoil spring  66 . As shown in  FIGS. 1 , and  3 , the charging handle body  88  includes a pair of laterally extending guide ribs  92  which slide along guide grooves  94  formed longitudinally on the inside of the upper receiver body  26 . The charging handle body  88  is elongated in order to completely close the charging handle openings  30  when in its forwardmost position. According to another feature of the invention, explained in further detail below, the charging handle lever  32  may be easily switched by the user from the left side to the right side through either charging handle opening  30 . The charging handle body  88  may be disassembled from the upper receiver body  26  by removing the charging handle lever  32  and aligning the ribs  92  with downward openings (not shown) in the guide grooves  94  at a rearward position beyond that normally encountered by manually cycling the action. 
         [0054]    Manual rearward cycling of the charging handle body  88  displaces the bolt carrier assembly  48  (via op rod  86 ) rearwardly, compressing the recoil spring  66 . As shown in this embodiment, a charging handle return spring  118 , carried by a guide rod  120 , may be included to bias the charging handle body  88  back toward a forward position, even when the bolt carrier assembly  48  is locked in an open, rearward position ( FIG. 28 ). As in the previously-described embodiment, the receiver body  26  may include both left and right charging handle openings  30  in order to allow the charging handle lever  32  to be switched between left-handed and right-handed operation orientation. The charging handle body  88  completely covers and closes both charging handle openings  30  when it is in its normal forward position. In this embodiment, the charging handle body  88  includes a central opening or bore  122  which receives the guide rod  120  when the charging handle body  160  is manually reciprocated rearwardly. 
         [0055]    Referring now to  FIGS. 1 ,  3 , and  6 , according to another feature of this embodiment, the receiver body  26  may include removable left and right accessory mounting rails  36 ,  38  corresponding to the three o&#39;clock and nine o&#39;clock positions. The rails  36 ,  38  may be attached with threaded fasteners  124  and may include a heat shield or insulating shim  126  (shown in  FIG. 6 ) to reduce heat transfer between the receiver body  26  and accessory mounting rails  36 ,  38 . If desired, the bottom rail  42  on the lower forearm cover  40  may be made as a separate piece and attached in a similar manner. Alternatively, the accessory rails  36 ,  38 ,  42 , may be made of a non-metallic, polymer material that is a poor thermal conductor to reduce heat transfer. 
         [0056]    Referring now to  FIGS. 5 and 6 , therein is shown longitudinal- and cross-sectional detail views of the gas block  74  and gas valve  78 . The gas block  74  includes a multi-position gas valve  78  which is rotationally adjustable and removable. The gas valve body  78  may be retained in any of the selected positions by an internal spring detent (not shown). When the gas adjustment control valve body  78  is removed, there is direct and axially-aligned access to the gas cylinder  80  for cleaning and maintenance. Also as described with respect to the previous embodiment, the gas block  74  may include a cleaning port  96  which is axially aligned with the gas port  72  of the barrel  70  and gas passageway  76 . This feature allows direct access to these passageways which may become fouled and are otherwise difficult to physically clean. 
         [0057]    As shown in  FIG. 7 , the bolt carrier assembly  48  includes a bolt carrier body  49  with an upwardly-extending forward bolt carrier lug  50  to which the tubular op rod  86  is attached. The bolt carrier body  49  includes upper left and right guide channels  54 ,  56  which slidingly engage longitudinal guide rails  58 ,  60  along the interior of the receiver body  26  (see  FIGS. 13 ,  21  and  22 ). The guide rails  58 ,  60  may be made of a hardened and wear-resistant material, such as steel, and may include a lubricious coating. If desired, exterior portions of the bolt carrier body  49 , particularly the guide channels  54 ,  56 , may also be provided with a lubricious coating. 
         [0058]    In the illustrated embodiment, the right guide channel  56  extends along a substantial portion of the overall length of the bolt carrier body  49 . The left guide channel  54  is interrupted between forward and rearward sections to allow for other mechanical structure and operations of the bolt carrier assembly  48 . These bolt carrier guide channels  54 ,  56  are positioned vertically between a longitudinal axis of the gas piston  82 , op rod  86 , forward bolt carrier lug  50 , and recoil spring  66  and the vertical axis of the barrel  70  and bolt  44 . Accordingly, recoil forces acting on the bolt carrier assembly  48  as it cycles resist tipping, which could cause uneven and undesirable wear and friction. 
         [0059]    Referring now also to  FIGS. 8 and 9 , therein is shown another feature of the present invention. Removal of the firing pin  46  is made easy by a vertically displaceable firing pin retention gate  128  that is retained by and slides in opposite vertical channels  130  provided in the bolt carrier body  144 . The retention gate  128  is actuated by a lift lever  132  which is mounted on a pivot pin  134  extending between the left and right guide channels  54 ,  56 . The lift lever  132  includes a tooth  136  that pivotally engages a window or socket  138  in the firing pin retention gate  128 . The lift lever  132  may be biased by a spring  140  into a closed position, as shown in  FIG. 8 . As shown in  FIG. 9 , when the lift lever  132  is pivoted (arrow  142 ), the tooth  136  is moved to lift (arrow  144 ) the firing pin retention gate  128  to a position allowing the firing pin  46  to be slid rearwardly out of the bolt  44 . This feature of the invention allows field removal of the firing pin  46  for disassembly of the bolt carrier  48  without risk of losing a small retainer pin, as used in the prior art. 
         [0060]    According to another aspect of the invention, the upper receiver assembly  12  can easily be manufactured and set up to eject spent casings to either the right or left side. Moreover, it can be made to be easily convertible by the user to operate in either a left-hand or right-hand mode. As previously described, the charging handle lever  32  is easily switchable from side to side. Additionally, a novel bolt design allows the user to selectively choose whether it is configured to eject spent casings toward the left or toward the right. 
         [0061]    Referring first to  FIG. 10 , therein is illustrated how the bolt  44  is easily removed in a forward direction from the bolt carrier body  49 . The firing pin  46  (not shown in  FIG. 10 ) is removed longitudinally, followed by removal of the bolt cam pin  146 , which allows the bolt  44  to be slid forwardly out of the bolt carrier body  49 . The bolt  44  can then simply be axially rotated 180° and reassembled into the bolt carrier body  49  with the bolt cam pin  146  and firing pin  46 . 
         [0062]    As shown in  FIGS. 11 and 12 , the geometry of the bolt head and face may differ significantly from the standard prior art bolt used in an AR15/M16 platform firearm. In the prior art bolt head  148 , the extractor  150  and ejector  152  are positioned to eject a spent casing at approximately a 2 o&#39;clock position (as viewed from the rear). The prior art bolt head  148  includes 8 radially-extending lugs, including one  154  carried by the extractor  150 . The bolt head  156  of the present invention also includes eight lugs  158  in a geometric orientation similar to that of the prior art bolt head  148 . However, the extractor  160  is positioned between lugs  158 . Thus, the extractor  160  and ejector  162  may be positioned at substantially horizontally opposed locations. Referring now again to  FIG. 10 , when the bolt is oriented as shown at  44   a , the extractor  160  and ejector  162  are oriented to eject toward the right (or three o&#39;clock position). When the bolt is rotated to the orientation shown at  44   b , the extractor  160  and ejector  162  are reversed and it is oriented to eject a casing toward the left (or nine o&#39;clock position). In either orientation, the bolt cam opening  164  is properly oriented to receive the bolt cam pin  146 . This is in contrast with the geometry of a prior art bolt head  148  ( FIG. 11 ) which, if reversed, would eject a casing toward an eight o&#39;clock position (as viewed from the rear). Thus, an ejection port is provided in the receiver body  128  at either the right  28   a  or left  28   b , or both (see  FIGS. 13 ,  16  and  17 ). Either or both ejection ports  28   a ,  28   b  are closed and covered by the lateral wall portions of the bolt carrier body  49  when in a closed, fully in-battery position. Accordingly, the same bolt  44  of the present invention can be repositioned, including by the user with simple field disassembly, to operate in either a left-handed or right-handed configuration. 
         [0063]    According to another aspect of the present invention, the upper receiver body  26  can be provided with both right and left ejector ports  28   a ,  28   b  that are reconfigurable, along with the bolt  44 , to select between right-handed or left-handed ejection. Referring now to  FIG. 13 , therein is shown is a rearwardly looking cross-sectional view through the upper receiver assembly  12  with the bolt carrier assembly  48  in an open or retracted position. This view illustrates the manner in which the upper receiver body  26  may be provided with both right and left ejector ports  28   a ,  28   b , making the upper receiver assembly  12  fully ambidextrous and easily converted in the field from right-to left-handed operation. As shown in  FIG. 13 , the bolt  44  is positioned such that the extractor  160  is positioned toward the right to provide ejection of spent casings through the right ejection port  28   a . If desired, the left ejector port  28   b  may be closed with an ejection port cover  166 . 
         [0064]    Referring now also to  FIGS. 14-17 , it can be seen that a shell deflector  168  may be provided rearwardly adjacent the open ejection port  226  to prevent ejected casings from exiting the port  28   a  at an angle more rearwardly than desired. Both the ejection port cover  166  and shell deflector  168  are secured to the upper receiver body  26  by a pair of interchangeable threaded fasteners  170 . In this manner, after the bolt  44  has been positioned, as described above, for left- or right-handed ejection, the ejector port cover  166  and shell deflector  168  are accordingly positioned on the receiver body  26  adjacent the respective ejector ports  28   a ,  28   b.    
         [0065]    Referring now to  FIG. 18 , therein is shown a partially cut-away detail top plan view of the engagement between the charging handle lever  32  and charging handle body  88 . A transverse window  172  is provided through a forward portion of the charging handle body  88 . A retainer pin  174  extends vertically through the transverse window  172 , secured to upper and lower portions of the charging handle body  88 . Axially rearward of the retainer pin  174  is a forwardly-directed spring-biased detent pin  176 . At the attachment end of the charging handle lever  32 , there is a partially open engagement bight  178  which is positioned to engage the retainer pin  174 . Against the spring bias of the detent pin  176 , the charging handle lever  32  can be pivotally positioned to a forward, folded position or a rearward, use position. When in the in-use position, an abutment surface  178  of the lever  32  bears against the charging handle body  88  and provides a fulcrum against which force can be applied to rearwardly cycle the charging handle body  88 . When the charging handle lever  32  is pivoted to the forward, folded position, it remains retained against the retainer pin  174  by the spring-biased detent pin  176 . To remove the charging handle lever, it can be manipulated rearwardly against the detent pin  176  without pivoting about the retainer pin  174 . In this manner, the bight  178  is disengaged from the retainer pin  174  and may be laterally removed from the transverse window  172 . The charging handle lever  32  may then be inverted and inserted from the opposite side of the transverse window  172  for pivotal engagement with the retainer pin  174  on the opposite side. 
         [0066]    As described above with reference to  FIGS. 10-18 , it can be seen that the upper receiver assembly  12  of the present invention can be fully ambidextrous and easily switched with few or no tools from right-handed to left-handed operation. The left or right position of the charging handle lever  32  can be chosen independently of the ejection direction of the bolt  44 . 
         [0067]    Referring now  FIGS. 19-22 , therein is shown a series of views illustrating the process of field disassembly or field stripping of the upper receiver assembly of this embodiment of the present invention. These figures show a side plan view of a rear portion of the upper receiver assembly  12  with the upper receiver body  26  and lower forearm cover  40  cut away in longitudinal section. For clarity of illustration, the recoil spring  66  and charging handle return spring  118  are not shown in their full length in these figures. In the case of the recoil spring  66 , it should be understood that the spring extends into the hollow op rod  86 . It is also to be understood that field stripping can be accomplished while the upper receiver assembly  12  is attached to a lower receiver  14  and tipped open on the forward pivot pin  24  or while the upper assembly  12  is completely separated from the lower receiver  14 . 
         [0068]    Referring first to  FIG. 19 , therein is shown at  180  a fixed portion of the recoil buffer. While the receiver body  26  is preferably milled from a lightweight aluminum alloy, the buffer fixed portion  180  is preferably made of a harder material, such as steel. The fixed portion  180  may be configured to fit in engagement slot (not shown) formed within the receiver body  26  and either permanently fixed or rigidly fixed, such as by a set screw  182 . The buffer fixed portion  180  includes a downwardly and forwardly directed tooth  186  configured to engage in a mating tooth  188  of a removable portion  184  of the recoil buffer system. The buffer removable portion  184  is secured to the rear closure plate  116  and recoil spring guide rod  68 . When the upper receiver assembly  12  has been either tilted open on pivot pin  24  or separated from the lower receiver  14  (as shown in  FIGS. 19-22 ), the recoil spring and buffer system may be removed from the receiver body  26  by applying forward force (arrow  194 ) against the rear cover plate  116  and against the force of the recoil spring  66 , which otherwise retains the removable portion  184  of the buffer engaged against the fixed portion  180 . If desired, the buffer system may be further secured against inadvertent displacement by use of a small but strong magnet  190  (such as a 0.25 inch diameter×0.10 inch thick rare earth magnet) set into a recess  192  in the buffer fixed portion  180 . Optionally, a locking mechanism in the form of a captured cross pin  200 , similar in design to a take-down pin  22  or pivot pin  24  may be provided on the upper receiver body  26  and extend through a transverse opening  202  in the rear closure plate  116  to prevent inadvertent dislodgement of the removable buffer block  184 . 
         [0069]    As shown in  FIG. 20 , once the respective teeth  186 ,  188  of the fixed and removable portions  180 ,  184  of the buffer have been disengaged from each other by the forward movement (arrow  194 ) and a slight downward movement (arrow  196 ), the entire recoil buffer assembly, including removable buffer portion  184 , rear cover plate  116 , guide rod  68 , and recoil spring  66  may be rearwardly pulled (arrow  198 ) from the receiver body  26 . 
         [0070]    As shown in  FIG. 21 , the bolt carrier assembly is now free to slide rearwardly along guide rails  58 ,  60 . In this manner, the entire bolt carrier assembly, including the bolt carrier body  49 , bolt  44 , firing pin  46 , op rod  86 , and piston  82 , easily slides rearwardly out the back of the receiver body  26 . 
         [0071]    Referring now to  FIGS. 23-25 , therein is shown the manner in which the barrel assembly  110  may be securely attached to the upper receiver body  26  and quickly detachable therefrom for exchange. The barrel assembly  110  includes a barrel trunnion  98  with a downwardly-extending lug  100  by which the trunnion  98  is directly secured to the forward pivot pin  24  of a standard lower receiver  14 . The barrel trunnion  98  includes a pair of laterally opposed, longitudinally extending engagement rails (or keys)  102 ,  104  which are configured to slidingly engage a rear pair of engagement grooves (or keyways)  106 ,  108  formed inside the receiver body  26 . 
         [0072]    The gas block  74  also includes a pair of laterally opposed, longitudinally extending engagement rails  204  which are configured to slidingly engage a pair of forward engagement grooves  206  formed on the interior of the upper receiver body  26 . 
         [0073]    Referring also again to  FIG. 6 , therein it can be seen that the relative exterior dimensions of the gas block  74  and interior dimensions of the upper receiver body  26  may be configured such that there is a minute gap between these members around most of the periphery of the gas block  74 , with engagement being predominantly or exclusively between the engagement rails  204  and engagement grooves  206 . In this manner, heat transfer between the gas block  74  and receiver body  26  (and lower forearm cover  40 ) is minimized. If desired, an upper engagement key or rail  208  may be provided on the gas block  74  and an upper engagement groove or keyway  210  provided at the forward end of the upper receiver body  128  to provide additional engagement stability. 
         [0074]    Referring particularly now  FIG. 25 , the lower forearm cover  40  may be secured to the upper receiver body  26  with captive threaded fasteners  112 . When the forearm cover  40  is secured in place, the barrel trunnion  98  is held in place by longitudinal engagement of the trunnion rails  102 ,  104  and rear engagement grooves  106 ,  108  and against longitudinal displacement by the lower forearm cover  40 . As in the previously-described embodiment, the barrel assembly  110  is firmly secured against lateral or vertical movement within the unitary upper receiver body  26 . If desired, the dimensions of a lower portion of the gas block  74  may be machined oversized (i.e., “proud”) in the range of 0.004 inches to 0.007 inches in order to assure that the lower forearm cover  40  holds it tightly in place. Although the barrel trunnion  98  is firmly secured against longitudinal displacement, the remainder of the barrel assembly  110  may freely expand and contract in the longitudinal direction as a result of thermal changes independent of the upper receiver body  26 . 
         [0075]    Referring now to  FIG. 26 , therein is shown a two-stage recoil spring and buffer system. A first, main recoil spring  212  is positioned on the guide rod  154  and extends into the hollow op rod  86 , much like that previously described with respect to other embodiments herein. A secondary buffer spring  214 , which is relatively significantly shorter and heavier, is situated forward of and axially aligned with the main recoil spring  212 . Between the springs  212 ,  214  is a block  216  which may act as a connector. 
         [0076]    Referring now also to  FIGS. 27 and 28 , it can be seen that, as the bolt carrier assembly  48  is driven rearwardly, either by force of gas pressure acting on the piston  82  (as shown) or manually retracted with the charging handle, the block  216  comes into contact with the end of the guide rod  68 . This limits and prevents further compression of the main recoil spring  212 , forcing further movement to compress the secondary recoil spring  214 . This final travel is limited to the very last portion of the recoil stroke, which may be no more than about 0.25 inches. Because the secondary recoil spring  214  is significantly stiffer than the main recoil spring  212 , a significant amount of recoil force is adsorbed in the final portion of travel and the velocity of the recoiling bolt carrier assembly  48  is quickly and significantly decelerated prior to impact contact between the forward lug  50  and the removable buffer block  184 . The heavier secondary spring  214  acts as a buffer, minimizing the impact of the bolt carrier assembly  48  against the lower buffer block  184 . 
         [0077]    This rapid adsorption of force is illustrated graphically in  FIG. 29 . At 0 (zero) position of the stroke, the bolt carrier assembly  48  is in battery, where the main recoil spring  212  is exerting about 4 pounds of force. As the action cycles rearwardly along the stroke curve, the force of the primary recoil spring  212  climbs until the block  216  contacts the end of the guide rod  68  and prevents any further compression of the primary spring  212 . The stroke curve (spring resistance force) climbs steeply in the final portion of the stroke. The deceleration (negative acceleration) of the bolt carrier assembly would be represented by a curve (not shown) substantially inverse of the force curve illustrated, with a significant portion of the deceleration occurring in a relatively small and final portion of the stroke cycle. 
         [0078]    Referring now to  FIGS. 30-33 , therein is shown at  218  a bolt according to another aspect of the invention in which the profile of the bolt lugs has an improved geometry. In most respects, the bolt  218  is the same in function and operation as that described above (at  44 ). It includes a bolt body  220  having a central longitudinal bore  222  for receiving the firing pin (not shown), a transverse bolt cam opening  224 , a spring-biased pivoting extractor  226 , and spring-biased ejector  228 . As described above with respect to the second embodiment, the illustrated bolt  218  has an extractor/ejector orientation that allows it to be reversed for left- or right-handed ejection of spent cartridge casings. This includes that the extractor  226  is oriented circumferentially between bolt lugs  230 , rather than carrying a lug as is the case in a prior art AR15/M16 bolt (see  FIG. 11 ). The improved bolt lug geometry described herein may be incorporated successfully into either style of bolt. 
         [0079]    Referring now in particular to  FIG. 33 , it can be seen that the peripheral profile of the bolt face formed by the lugs  230  is a generally continuous curvilinear shape, rather than the angular, generally radial and circumferential shape of the prior art bolt  148  (see  FIG. 11 ) or reversible bolt  146  ( FIG. 12 ) described above. The outer ends of each lug are rounded to eliminate a sharp longitudinal edge. The spaces between lugs are likewise rounded as troughs at the base of each adjacent lug, eliminating a sharp cut at the base of each lug. Although this profile narrows the width of each lug at the outer end  232 , the amount of material at the base  234  where the lug joins the body of the bolt face is increased by 20-30%. Moreover, the elimination of sharp cuts at these locations greatly enhances the structural integrity of the bolt face and lugs  230  to resist shear forces and reduces stress points where cracking in hardened materials is most likely to originate and occur. An outline of one prior art lug profile is shown for comparison in phantom line at  236  in  FIG. 33 . The overall width (shown at  238 ) of the lug  230  is maintained to be substantially that of the prior art lug  236 . The curvature of the outer end  232  may have a radius (shown at  240 ) substantially half the width (shown at  238 ) of the lug  230 . Likewise, the overall depth (shown at  242 ) of each lug  230  is maintained to be substantially that of the prior art lug  236 . The curved profile of the space between adjacent lugs  230  may have a radius (shown at  244 ) that is substantially half the circumferential distance between the lugs  230  such that the smoothest possible curvilinear transition is provided to the profile. 
         [0080]    Many features have been listed with particular configurations, options, and embodiments. Any one or more of the features described may be added to or combined with any of the other embodiments or other standard devices to create alternate combinations and embodiments. Although the examples given include many specificities, they are intended as illustrative of only one possible embodiment of the invention. Other embodiments and modifications will, undoubtedly, occur to those skilled in the art. Thus, the examples given should only be interpreted as illustrations of some of the preferred embodiments of the invention, and the full scope of the invention should be determined by the appended claims and their legal equivalents.