Patent Publication Number: US-11022385-B2

Title: Operating system for small caliber rifles

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claim benefit under 35 U.S.C. § 121 as a division of U.S. patent application Ser. No. 15/720,522, titled “OPERATING SYSTEM FOR SMALL CALIBER RIFLES,” filed on Sep. 29, 2017, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/402,198, titled “OPERATING SYSTEM FOR SMALL CALIBER RIFLES,” filed on Sep. 30, 2016, all of which are hereby incorporated by reference in their entireties for all purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to firearms. Specifically, the present disclosure is directed to operating systems for small arms, such as semiautomatic rifles. 
     BACKGROUND 
     Firearms design involves many non-trivial challenges. In particular, projectile weapons, such as small caliber rifles, have faced a challenge to produce firearms that are more durable and have improved operational efficiency. 
     SUMMARY 
     The system and method described in the present disclosure meets one or more of the above needs by providing an operating system for small caliber rifles which may include one or more of a pivoting interface between the op-rod and the bolt carrier, a keyway for improving stability in a direction perpendicular to the direction of firing, a firing pin lock which may include a firing pin lever and a firing pin latch, a recoil assembly including a plunger and one or more plunger return springs, a tapered lug which may include an angled interface at an aft end, an elastomeric insert positioned at a cone interface, and/or a carrier assembly which may include at least a recoil assembly key slot, a firing pin lever guide slot, a firing pin retention pin, a recoil assembly rotation slot, and a harmonic cam. Firearm, as used herein, may refer to a rifle. Firearm, as used herein, may also refer to a small caliber rifle such as the SIG SAUER® MCX or MPX. Carrier, as used herein, may refer to a bolt carrier. 
     In one aspect, a bolt assembly for a rifle is disclosed. In one embodiment, the bolt assembly includes a bolt having a bolt body with a distal end portion, and a lug extending radially outward from the distal end portion. The lug has an outer lug surface generally parallel to the bolt body. The lug also has lug sides and an aft portion extending between the outer lug surface and the bolt body. A cross sectional size of the lug decreases as the lug extends from the bolt body to the outer lug surface. The distal end portion of the bolt is constructed to be received by a breech-end of a barrel of the rifle. 
     In some embodiments, the cross sectional width is measured from a first lug side to the opposed lug side on the same lug, the cross sectional width being substantially perpendicular to the major axis of the rifle. 
     In other embodiments, the cross sectional width is measured from the aft surface of the lug to the forward surface of the lug, the cross sectional width being substantially parallel to a major axis of the rifle. 
     In yet other embodiments, a first cross sectional width between the lug sides and a second cross sectional width between the aft and forward surfaces both decrease as the lug extends from the bolt body to the outer lug surface. 
     In some embodiments, the outer lug surface includes a chamfer or rounding. 
     In another embodiment, at least one of the lug sides extends toward the bolt body at an average angle of at least 1°. 
     In some embodiments, at least one of the lug sides has a first sloped portion defining a first angle and a second sloped portion defining a second angle, wherein the first angle is at least 30 and the second angle is at least 1° with respect to a radius of the bolt extending to a center of the outer lug surface. In other embodiments, both lug sides have the first and second sloped portions. 
     In another embodiment, one or both lug sides are arcuate from the outer lug surface to the bolt body. In another embodiment, the aft portion is arcuate from the outer lug surface to the bolt body. 
     In another embodiment, the aft portion extends toward the bolt body at an average angle from 5° to 20° with respect to a perpendicular extending from the bore axis. 
     Another aspect of the present disclosure is directed to an operating system for a small caliber rifle. In one embodiment, the operating system includes a carrier assembly, a recoil assembly operatively connected to the carrier assembly and including an op-rod of the rifle, and a bolt assembly with a bolt retained by the carrier assembly. A connection between the carrier assembly and the recoil assembly is a pivoting interface. 
     In another embodiment, the pivoting interface is configured to decouple non-axial motion of the recoil assembly from axial motion of the carrier assembly. 
     In another embodiment, the pivoting interface is positioned toward a rear end of the op-rod. 
     In another embodiment, the pivoting interface comprises a recoil assembly rotation slot in a top surface of the carrier assembly, where the recoil assembly rotation slot has a concavely-rounded surface configured for rotation therein of a corresponding convexly-rounded extension on the recoil assembly. 
     In another embodiment, the pivoting interface comprises a rounded interface between the recoil assembly and the carrier assembly, where the rounded interface is configured to allow free rotation of the recoil assembly with the carrier assembly about a median plane of the small caliber rifle. 
     In another embodiment, the pivoting interface is configured to direct recoil energy downward and toward an aft end of the small caliber rifle. 
     In another embodiment, the pivoting interface is configured to reduce at least one of (i) pitch excitation of the carrier assembly, (ii) torque on the carrier assembly, and (iii) moment forces during firing of ammunition. 
     In another embodiment, the carrier assembly is constructed for removal of the bolt by maintaining the firing pin retention pin in connection with the carrier assembly during removal of the bolt. 
     In another embodiment, the operating system for a firearm includes a firing pin and a recoil assembly that includes an op-rod and a firing pin lever attached to the op-rod. The firing pin lever is configured to prevent the firing pin from moving forward to strike an ammunition primer unless a trigger is in the firing position. 
     In another embodiment, the firing pin lever is displaced by a hammer of the firearm in response to the trigger being pulled to the firing position, thereby allowing the firing pin to move forward. 
     In another embodiment, a plunger return spring is configured to bias the firing pin toward a resting state. 
     In another embodiment, an operating system for a firearm includes a firing pin configured to strike a primer upon depression of a trigger, where the firing pin includes a camming surface oriented so that the firing pin is biased toward a rearward resting state. The firing pin also has an aft portion defining a stop surface. 
     In another embodiment of an operating system for a firearm, the operating system includes a lower receiver with a recess defined in an aft portion, a carrier group with an aft end defining an aft recess; an elastomeric insert positioned between the aft end of the carrier group and the recess defined in the aft portion of the receiver, where the elastomeric insert is configured to absorb recoil forces utilizing a conical or frustoconical interface. 
     In another embodiment, the elastomeric insert defines a first outer and a second outer portion, the first outer portion being conical and the second outer portion being cylindrical. 
     In another embodiment, the aft recess in the carrier group has a frustoconical shape. 
     Another aspect of the present disclosure is directed to a firearm gas system including a piston defining a cavity therein, where the cavity has a volume greater than 50% of the volume of the piston, and where the piston is operatively coupled to an op-rod of the firearm. 
     Yet another aspect of the present disclosure is directed to a method for operating small caliber rifles. In one embodiment, the method includes securing a firing pin in a resting state engaged by a firing pin catch, pulling a trigger to disengage the firing pin catch from the firing pin, releasing the firing pin to strike a primer, and returning the firing pin to a resting state using one or more springs. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been selected principally for readability and instructional purposes and not to limit the scope of the disclosed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  illustrates a left-side elevational view of a bolt distal end portion showing a set of tapered lugs in accordance with one embodiment of an operating system for small caliber rifles of the present disclosure. 
         FIG. 2  shows a front-end view of a bolt assembly of the operating system for small caliber rifles in accordance with the present disclosure. 
         FIG. 3  shows a top, left-side, and rear perspective view of a carrier assembly of the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 4  shows a top and front perspective view of a rifle lower receiver in accordance with one embodiment of the operating system for small caliber rifles of the present disclosure. 
         FIG. 5  shows a top and front view of an elastomeric insert for absorbing shock in accordance the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 6A  shows a bottom and rear perspective view of a conventional carrier assembly as known in the art. 
         FIG. 6B  shows a bottom and rear perspective view of a carrier assembly and firing pin of the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 7  shows a top, left-side, and front perspective view of a carrier assembly and recoil interface body of the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 8  shows a top and left-side perspective view of the carrier assembly of  FIG. 7  showing a recoil assembly key slot in accordance with an embodiment of the present disclosure. 
         FIG. 9  illustrates a top, left-side, and front perspective view of a recoil assembly and bolt carrier group of the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 10  shows a top, left-side, and rear perspective view of a recoil assembly of the operating system for small caliber rifles in accordance with an embodiment of the present disclosure. 
         FIG. 11A  shows a conventional mechanism for mounting a recoil assembly to a carrier assembly using a dovetail join as known in the art. 
         FIG. 11B  shows a top, left-side, and rear perspective view of a recoil assembly with part of a pivoting interface of the operating system for small caliber rifles in accordance with an embodiment the present disclosure. 
         FIG. 12  illustrates a left-side elevational section of part of a fire control group and recoil assembly of the operating system for small caliber rifles in accordance with one embodiment the present disclosure. 
         FIG. 13  shows a left-side elevational view of a firing pin of the operating system for small caliber rifles in accordance with one embodiment of the present disclosure. 
         FIGS. 14A-14C  show a firing pin lock and fire control group of the operating system at various stages of operation, in accordance with an embodiment of the present disclosure. 
         FIG. 15  illustrates a side elevational view of a hollowed-out piston and gas system in accordance with an embodiment of the present disclosure. 
         FIG. 16  illustrates a cross-sectional view of a hollowed-out piston and gas system in accordance with an embodiment of the present disclosure. 
         FIG. 17  shows a side elevational view of a firearm, in one embodiment of the disclosure. 
         FIG. 18  shows a graphical representation of recoil force vs. time for a firearm configured with a cone elastomer buffer as compared to a firearm configured with a flat elastomer buffer. 
     
    
    
     These and other features of the present embodiments will be understood better by reading the following detailed description, taken together with the figures herein described. The accompanying drawings are not intended to be drawn to scale. For purposes of clarity, not every component may be labeled in every drawing. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     In conventional operating systems for semiautomatic and automatic rifles, some drawbacks include frictional wear and joint failure which may result from extended use. Joints for use at a connection between a recoil assembly and a carrier assembly include, for example, dovetail joints (see  FIG. 11A ) that retain the recoil group generally parallel to the bolt carrier. During recoil, dovetail joints may have a large moment force applied to the structural components in the fire control group and, as a result, are subject to wear with extended use. Accordingly, it is desirable to provide an improved operating system for small caliber rifles that is more durable and less prone to component wear with extended use. As used herein, the term “small caliber” refers generally to ammunition commonly used in small arms, such as 300 BLK, 5.56×45 mm, .223 REM, 7.62×39 mm, 7.62×51, and 308 WIN to name a few examples of standard rifle cartridges. The term “small caliber” also includes pistol caliber carbines, which may be chambered in 9 mm Luger, .40 S&amp;W, .45 AUTO and other suitable cartridges to name a few examples. The present disclosure is not limited to these examples and is contemplated for use with firearms of other calibers and ammunition types. 
     In firearms, a safety or safety catch is a mechanical device used to help prevent the unintentional discharge of a firearm. Safeties can generally be classified as internal safeties or external safeties. Internal safeties typically do not receive input from the user. In contrast, external safeties typically require user input, for example, by toggling a switch between “safe” and “fire.” Another type of external safety in some firearms is an integral locking mechanism that must be deactivated by the user with a unique key before the gun can be fired. These integral locking mechanisms are intended as child-safety devices for use during unattended storage of the firearm—not as safety mechanisms while carrying or using the firearm. 
     Safety mechanisms include, for example, safety wing, action release, hammer block safety, trigger block safety, bore lock safety, grip safety, and a trigger lock safety. For example, trigger locks are an external device installed in the trigger guard that physically prevent the trigger from being pulled to discharge the weapon. Trigger locks generally have two pieces extending from either side of the lock that come together behind the trigger to obstruct the trigger movement. Trigger locks may be locked in place and unlocked with a key or combination. In order to be effective, however, trigger locks require the user to install the trigger lock in the trigger guard and place it in a locked condition. Since trigger locks require user action and are not built into the mechanical structure of the firearm itself, a trigger lock may be less likely to prevent unintentional discharge of the firearm. Also, trigger locks are generally designed to be used during firearm storage, not when the firearm is in use or carried by the user. 
     Another example of a safety mechanism is a lever on a trigger, or trigger safety. The trigger safety is a type of device designed to prevent unintentional discharge when the firearm is in use, such as when the firearm is carried on the user or in the user&#39;s hand. The lever must be manually depressed before the trigger can be moved to cause movement of a trigger bar to discharge the firearm. However, when the user&#39;s finger is on the trigger with the lever depressed, the firearm may still discharge unintentionally if the user&#39;s hand or body is bumped since a slight movement of the trigger may be sufficient to fire the firearm. 
     With semiautomatic rifles, such as rifles based on the AR-15 platform, for example, the rifle typically includes a safety switch that includes a pin extending through the receiver. The pin has a recess or flat on one side. In the “safe” position, the pin blocks the trigger from being pulled, in the “fire” position, the trigger clears the recess in the pin and can be pulled to discharge the firearm. In such rifles, however, the firing pin may be free floating, with no safety mechanism acting on the firing pin. When chambering a round, the charging handle is pulled rearward to draw the bolt out of the chamber and allow a cartridge to be positioned for feeding to the breech. Upon releasing the charging handle, the bolt slides forward to battery and chambers the round. When the bolt is cycled forward and locked, the firing pin has the potential to move forward and hit the cartridge&#39;s primer, possibly resulting in an unintended discharge. Accordingly, when the bolt is released from the bolt catch while chambering a round, and prior to pulling back on the trigger, an unintentional discharge may result. Thus, there is a need for an improved firearm design configured to reduce unintended firing. 
     Embodiments of the present disclosure attempt to overcome limitations of certain safety mechanisms known in the art and relate to an apparatus and method for limiting or preventing unintentional discharge of a firearm. Embodiments of the present disclosure also relate to an apparatus and method for an improved operating system for small caliber rifles. Numerous configurations and variations will be apparent in light of this disclosure. 
     As will be seen, the devices and methods taught herein offer an improvement to firearm safety, particularly as applied to semiautomatic and automatic rifles, carbines, submachine guns, and machine guns. The devices and methods disclosed herein are intended to avoid unintentional discharge, as well as the consequences of firearm malfunctions. Improving firearm safety may help to eliminate or minimize the risks of unintentional death, injury or damage caused by improper handling of firearms. The devices and methods taught herein may help to improve firearm safety, including results of drop safety tests. 
     As will be seen, the devices and methods taught herein offer an improved operating system for small caliber rifles. Pursuant to one aspect of the present disclosure, there is contemplated a firearm comprising a firearm receiver, a fire control group installed in the firearm receiver and comprising a carrier assembly, a recoil assembly, and a bolt assembly. The carrier assembly houses the bolt assembly and the recoil assembly may be positioned to engage the carrier and bolt assemblies during operation of the firearm. The operating system may contain an operation rod, herein referred to as an op-rod, which is pushed by a gas system during firing. During firing, gas pressure pushes the op-rod rearward to move the bolt carrier group rearward against spring forces. Oscillations may be induced through the gun, causing pitch excitation. In particular, if any components of the fire control group are misaligned, pitch excitation may occur as a result of a conventional connection between the carrier assembly and the recoil assembly (e.g., a dovetail joint as shown in  FIG. 11A ). Pitch excitation, as used herein, may be defined as movement or oscillations of the recoil assembly in a harmonic manner (e.g. flopping up and down or whipping around following firing). When pitch excitation occurs, it may be harder to eject the ammunition casing off of the bolt face since the recoil group may deviate from being parallel to the bore axis and apply torque to the bolt carrier group. The design as described herein eliminates or reduces pitch excitation by, for example, allowing rotation of the recoil assembly with respect to the carrier assembly at the pivoting interface. As a result, the recoil assembly applies less torque to the carrier group and more efficiently moves the bolt carrier group along the bore axis. 
     In the present disclosure, a pivoting interface is added toward a rear end of the op-rod, which is configured to absorb torque and eliminate moment forces which may otherwise cause friction load losses, joint failure, and/or wear to firearm components. In some cases, failure rates may be improved by a factor of three times, four times, or five times over conventional operating systems. The pivoting interface is configured to decouple non-axial motion in the recoil assembly from axial motion in the carrier assembly. At least in part due to the eliminating or reducing of moment forces resulting from firing, the pivoting interface is configured to provide a more consistent bolt-cartridge interface during firing, a firearm that operates with a smaller required gas volume to cycle the action, and a firearm that operates more efficiently using less energy. It should be noted that, while generally referred to herein as a ‘pivoting interface’ for consistency and ease of understanding the present disclosure, the disclosed pivoting interface is not limited to that specific terminology and alternatively can be referred to, for example, as a hinge joint or other terms. 
     The pivoting interface may comprise a recoil assembly rotation slot in the carrier assembly and a corresponding rounded surface on the recoil assembly. The rounded surface may be configured to rotate in the recoil assembly rotation slot following firing. It should be noted that, while generally referred to herein as a ‘recoil assembly rotation slot’ for consistency and ease of understanding the present disclosure, the disclosed recoil assembly rotation slot is not limited to that specific terminology and alternatively can be referred to, for example, as a keyway or other terms. As will be further appreciated, the particular configuration (e.g., materials, dimensions, etc.) of the pivoting interface and recoil assembly rotation slot configured as described herein may be varied, for example, depending on whether the target application or end-use is military, tactical, or civilian in nature. Numerous configurations will be apparent in light of this disclosure. 
     Embodiments of the present disclosure also relate to an apparatus and method for a firing pin lock. In some embodiments, the apparatus and methods taught herein offer a firing pin lock attached to an op-rod in the recoil assembly and comprising a firing pin catch. The firing pin catch may be configured to prevent the firing pin from moving forward and hitting the primer when not intended. In some embodiments, the firing pin catch may interface with the hammer during firing. The firing pin catch, the firing pin lever, or both may define a stop surface that engages the head of the firing pin. The firing pin catch, the firing pin lever, or both may prevent unintentional discharge of the firearm. When a user pulls back on the trigger, the firing pin lever is rotated, thus releasing the firing pin catch from engagement with the firing pin and causing release of the firing pin to strike the primer. In some embodiments, a return spring, a camming surface, or both may be included to bias the firing pin toward a resting state. In some embodiments, following release of ammunition, the return spring, the camming mechanism, or both, bias the firing pin back into a resting state. 
     In the present disclosure, an op-rod can be coupled to the bolt carrier. In some embodiments, a short stroke piston is used. With a short-stroke or tappet system, the piston moves separately from the bolt assembly. The piston may directly push the carrier assembly parts, such as in the M1 carbine, or it may operate through a connecting rod or assembly, such as in the Armalite AR-18 or the SKS rifle. In either case, the energy is imparted in a short, abrupt push and the motion of the gas piston is then arrested to allow the carrier assembly to continue through the operating cycle using kinetic energy. The short-stroke piston has the advantage of reducing the total mass of recoiling parts compared to the long-stroke piston. This, in turn, enables better control of the weapon, compared to firearms with long-stroke counterparts due to less mass needing to be stopped at either end of the bolt carrier travel. 
     Periodically with firearms, and in particular for firearms equipped with sound suppression, there is a need to periodically clean out built up carbon deposits in the firearm. With many firearms, it is necessary to perform several steps to remove the bolt assembly. For example, to remove the bolt, it may be necessary to first remove individual parts (i.e. a spring) and push in a pin. The embodiments disclosed herein can provide easy disassembly for cleaning, and can eliminate the need to remove or maneuver individual parts thereby creating a stronger bolt assembly interface. The embodiments disclosed herein eliminate the need to physically remove firing pin retention pin  58  from carrier assembly  40  prior to removal of the bolt assembly. Firing pin retention pin  58  is retained in connection with carrier assembly  40  and thereby reduces the potential for lost parts. Bolt assembly  30  is removed by disassembling interlocking components. Bolt  140  can be removed by sliding first pin retention pin  58  outwards (firing pin retention pin  58  is captured by carrier assembly  40  and does not become a loose part), removing firing pin  41  axially out a rear end of bolt assembly  30 , removing a cam pin out of a cam pin receptacle, and pulling bolt  140  forward out of carrier assembly  40 . Many of the embodiments disclosed herein are configured for a user to remove the bolt by removing the bolt in one assembly unit. The designs disclosed herein can take advantage of a reduced part count and provide for more efficient assembly and disassembly. 
     Turning now to the drawings example embodiments of the present teachings are discussed. 
     Bolt Assembly/Tapered Lug 
     Referring now to  FIGS. 1-2 , an elevational view and a front-end view, respectively, illustrate a bolt assembly  30  in accordance with an embodiment of the present disclosure. Bolt assembly  30  has a bolt  140  extending along a bore axis  5  to a forward end  93  that includes a set of radially extending lugs  144 . Lugs  144  extend radially outward from an outside surface  98  of bolt  140  and are distributed circumferentially along outside surface  98 . As best shown in  FIG. 2 , a bolt face  99  is defined radially inside the plurality of lugs  144  at forward end  93  of bolt body  151 . Bolt face  99  defines a firing pin opening  100  for operation of firing pin  41  along the bore axis  5 . In conventional bolt assemblies, the bolt lug can crack due to fatigue from extended use. As bolt  140  rotates, clearance is lost between lugs  144  and corresponding slots at the breech, resulting in wear and breakdown of components. In contrast, the design described herein incorporates tapering on one or both sides  64  of each lug  144  to increase clearance with slots in the breech and extend lifetime of the parts. 
     In one embodiment, each of the plurality of lugs  144  has sides  64  extending from a radial outside lug face  101  to outside surface  98  of bolt  140 . Each of the plurality of lugs  144  can be tapered by a first angle α and/or a second angle β as described below. In one embodiment, side(s)  64  of each of lugs  144  define a side taper angle β from 5 to 20 degrees with respect to a radius extending from the center of bolt  140 . Other values of side taper angle β are acceptable, including 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or 19 degrees. One or both sides  64  of each lug  144  can be tapered. In one embodiment, bolt  140  rotates according to the right-hand rule, so one side  64  of each lug is more prone to contact slots of the breech when moving into and out of battery. Thus, only one side  64  of each lug  144  may be tapered. Similarly, an opposite side  64  of lugs  144  is more prone to contact slots of the breech when bolt  140  rotates in an opposite direction. In another embodiment, each side  64  lugs  144  has a different side taper angle β, such as when one side  64  of a lug  144  has an increased side taper angle β compared to the other side  64  of the same lug  144 . 
     In addition to or as an alternative to side taper angle β, aft portion  147  of lugs  144  may be provided with an aft taper angle α. Aft portion  147  of lugs  144  can extend in a straight line, stepped, or a curve from outer lug surface  101  to bolt body  151 . Radial outer lug surface  101  can be rounded, flat, can include a chamfer or edge rounding. In some embodiments, lugs  144  can be curved extending from bolt body  151  to radial outer lug surface  101 . In one embodiment, aft portion  147  extends generally in a straight line and has aft taper angle α that is greater than 90° to about 120° with respect to bore axis  5 . Stated differently, aft taper angle α is from 1° to 30° with respect to an axis perpendicular to bore axis  5  in some embodiments. 
     In some embodiments, the side taper angle β and/or aft taper angle α may be constant along the taper. Alternatively, side taper angle β and/or aft taper angle α may vary along the length of the taper. Aft taper angle α may be at a constant angle of between 1° and  200 , between 12° and 18°, between 14° and 16°, or between 5° and 15° with respect to an axis perpendicular to bore axis  5 . Alternatively, aft portion  147  of lugs  144  can be curved or stepped and the aft taper angle α may vary from an angle of between 0° and 90°, between 10° and 70°, or between 14° and 60° with respect to an axis perpendicular to bore axis  5 . Side taper angle β may be a constant value from 1° to 20°, from 3° to 7°, or from 4° to 5° with respect to an axis perpendicular to bore axis  5 . Alternatively, sides  64  of lugs  144  can be curved or stepped and angle β may vary from an angle of between 0° and 90°, between 10° and 70°, or between 14° and 60° with respect to the radius of bolt  140 . In some embodiments, the radius of the curvature may be between ¼″ and ½″. At different points along the sides  64  of each lug  144  or along the aft portion of lugs  144 , the radius can vary. 
     Lug  144  with tapered sides  64  and/or tapered aft portion  147  is configured to provide decreased stress and/or reduced wear resulting in a longer life of the bolt assembly  30  and operating system  10 . Lug  144 , when provided with tapered sides  64 , is provided with the advantage of increased clearance and reduced friction with the extractor and/or with the complimentary shaped set of teeth (i.e. mating recesses) in the barrel. The design disclosed herein reduces friction between the set of lugs  144  on bolt assembly  30  and a set of teeth in the barrel caused during rotation. Functionally, sides  64  and/or aft portion  147  of lug  144  may be configured to increase an operating clearance with mating recesses or slots at a breach end of the barrel. 
     Carrier Insert 
     Referring now to  FIGS. 3-5, and 6A-6B , carrier assembly  40  is illustrated in accordance with another embodiment of the present disclosure.  FIG. 4  shows receiver  75 , including circularly shaped contact point  70 .  FIG. 5  shows a component view of an example elastomeric insert.  FIG. 6B  illustrates carrier assembly  40 , including conical region  66  for mating with insert  60 . For comparison,  FIG. 6A  illustrates a rear perspective view of an aft portion of a carrier of the prior art. 
     As disclosed herein, carrier  38  has an aft end  96  defining a conical region  66  and an insert  60  shaped to mate with conical region  66 . Conical region  66  may be formed in an aft end  96  of carrier  38  (shown in  FIGS. 3 and 6B ). Conical region  66  may be formed with several individual wing shaped regions, wider at an aft end and narrower at a forward end. For example, conical region  66  may be a single wing shaped region or may be formed with 2, 3, 4, 5, or more individual wing shaped regions. Insert  60  (shown in  FIG. 5 ) may be formed to have a conical or frustoconical shape to mate correspondingly with conical region  66 . Insert  60  may be placed in conical region  66  between aft end  96  of carrier assembly  40  and contact point  70  of a lower receiver (shown in  FIG. 4 ). Insert  60  can decrease the stopping force of the carrier  38  from recoil forces, reducing internal loads to the carrier assembly  40  and reducing the overall dynamic load. Insert  60  can provide a greater surface area at the interface between carrier assembly  40  and receiver  75  to allow for gradual deceleration of carrier assembly  40  as insert  60  is compressed. 
     Insert  60  may comprise an elastomer, a polymer, synthetic rubber, or the like, and may be configured to absorb energy, reduce recoil, or both. In various embodiments, insert  60  can have a Shore A hardness of from 15 to 110, and more specifically, from 90 to 100. Insert  60  is configured to provide for a smooth operation of firearm  200  by spreading recoil forces over a larger surface area, reducing the rate of deceleration, and absorbing recoil forces to result in a gentler impact.  FIG. 18  illustrates the reduction in recoil forces vs. time of a conical or frustoconical shaped insert as compared to recoil forces vs. time for a flat insert. The peak force for the conical or frustoconical insert is shown as being about 260.9 lbf, while the peak force for the flat insert is shown as being about 367.6 lbf. In this example, the recoil forces have been shown to be reduced by about 29%. Different configurations may yield variations in exact recoil force reduction. Recoil force may be reduced by 10%, 20%, or 30% by utilizing the embodiments described herein. It should also be noted that a time delay is measured for the onset of detectable recoil force in the conical insert vs. the flat insert of 0.0007 seconds. Thus, the conical insert is configured to yield both a reduction in total recoil force and a delay in the onset of detectable recoil force as compared to the flat insert. Insert  60  is configured to spread and expand radially as force is applied axially thereto. Insert  60 , conical region  66 , and contact point  70  (shown in  FIG. 4 ) are configured to provide a mechanical damping effect as the action of firearm  200  is cycled. Contact point  70  can be a circular recess in an aft end of the lower receiver  75 . An inside circumference of insert  60  (shown in  FIG. 5 ) may be formed with a stepdown region (i.e. with one region having an inside circumference greater than another region). Insert  60  can be formed with a flat surface perpendicular to bore axis  5  at aft end  142  of insert  60 . A first portion  148  of insert  60  may be formed with an outside surface having a conical or frustoconical shape. A second portion  149  of insert  60  may be formed with an outside surface having a cylindrical shape. 
     Pivoting Interface 
     Referring now to  FIGS. 7-8 , top and left-side perspective views illustrate carrier assembly  40  in accordance with an embodiment of the present disclosure.  FIGS. 7-8  show carrier assembly  40  and recoil assembly  20  and how the two interact with each other, including by pivoting interface  61 .  FIG. 7  includes a recoil interface body  67  installed on bolt carrier  38 . In one embodiment, carrier  38  and recoil interface body  67  define a pivoting interface  61 . In one embodiment, carrier  38  defines a recoil assembly rotation slot  57  configured to receive a rounded extension  46  on recoil interface body  67 . Carrier  38  also includes one or more recoil assembly key slot  56  a firing pin safety guide slot  45 , and a harmonic cam  55 . 
     In one embodiment, pivoting interface  61  includes an extension  46  with a convexly rounded surface  85  corresponding to a concavely rounded bottom portion  161  of recoil assembly rotation slot  57  defined in a top surface of carrier  38 . Recoil assembly rotation slot  57  has a rounded profile that extends partially through carrier  38  in a direction perpendicular to bore axis  5 . As such, recoil assembly rotation slot  57  has an open top and defines a flat surface  160  at a blind end and rounded bottom portion  161 . 
     Pivoting interface  61  may be positioned toward a rear end of the op-rod  50 , and acts as a connecting point transferring force between recoil assembly  20  and carrier assembly  40 . Pivoting interface  61  is configured to provide a consistent bolt-cartridge interface during firing, operates using a smaller required gas volume, and operates more efficiently since it requires less energy. Following firing of ammunition, forward end  91  of recoil assembly  20  may move in an upward direction due to recoil effects. Pivoting interface  61  directs recoil energy axially toward aft end  172  of firearm  200  and downward against carrier  38 . Rounded extension  46  allows free rotation of recoil assembly  20  along median plane  7  with respect to carrier assembly  40 , thereby eliminating pitch excitation of carrier assembly  40 . In other embodiments, pivoting interface  61  is hinged rather than a pivoting or rotating interface. In some embodiments, pivoting interface  61  is configured for rotational motion about a pivot point. In other embodiments, recoil interface body  67  can rotate about a pin located centrally in rounded extension  46 . 
     Recoil assembly rotation slot  57  may provide a surface on which rounded extension  46  pivots on recoil assembly  20 . Recoil assembly rotation slot  57  may be formed by cutting with a cylindrical die, for example, a region out of a top portion of carrier assembly  40 . Flat surface  160  or blind end may be formed at one lateral end, with a rounded recoil assembly rotation slot  57  extending laterally to the opposite lateral end, as shown in  FIG. 8 . 
     Functionally, pivoting interface  61  is configured to absorb torque and reduce moment forces resulting from recoil effects during firing. In doing so, pivoting interface  61  may provide stability in a direction perpendicular to bore axis  5  by preventing or reducing upward forces exerted on carrier  38  by recoil assembly  20 . Pivoting interface  61  therefore reduces frictional load losses, joint failure, and/or wear to firearm components. One advantage provided by the use of pivoting interface  61  described herein is that it allows the recoil assembly  20  to move more naturally in response to firing of ammunition and better transfers forces axially to carrier assembly  40 . 
     Recoil Assembly 
     Referring now to  FIGS. 9, 10, and 11B , a recoil assembly  20  is illustrated in accordance with an embodiment of the present disclosure.  FIG. 9  is a left-side perspective view illustrating details of components of an operating system  10  described herein and shows recoil assembly  20  assembled with carrier assembly  40  and bolt assembly  30  using pivoting interface  61  as discussed above. Some of the components illustrated in  FIG. 9  form part of fire control group  145  shown in  FIGS. 14A-14C  discussed below.  FIGS. 10 and 11B  illustrate right-side perspective views of recoil assembly  20  showing firing pin lever  105  pivotably attached to recoil interface body  67  with lever retaining pin  52 . For comparison,  FIG. 11A  illustrates a dovetail joint  135  as used in the prior art between carrier assembly  40  and recoil assembly  20 . 
     Recoil assembly  20  of the present disclosure is configured to absorb recoil forces and/or energy resulting from firing and move carrier assembly  40  axially rearward in response to cycle the action of firearm  200 . In one embodiment, recoil assembly  20  includes at least one return plunger  51  with a plunger return spring  47 , and one or more recoil springs  53  each installed along a guide rod  54 . Plunger return spring  47  and return plunger  51  are also illustrated in  FIG. 12 . A forward spring retainer  48  can be positioned on a forward end  91  of the recoil assembly  20 , and an aft spring retainer  49  can be positioned on an aft end  92 . The recoil assembly  20  also includes an op-rod  50 , a lever retaining pin  52 , and firing pin lever  105  with a firing pin catch  59 . Recoil assembly  20  can be configured with tabs  141  which engage recoil assembly key slots  56  (shown in  FIG. 3 ) in carrier assembly  40  to provide a stiff rotational interface that resists rotation of carrier assembly  40  about bore axis  5  when bolt  140  rotates. Bolt  140  rides in a short carrier  38  to allow translation and rotation of bolt  140  by harmonic cam  55  without lost motion. The connection between tabs  141  and key slots  56  (shown, for example, in  FIG. 9 ) prevents the bolt  140  from hitting the chamber wall while entering or exiting the chamber of firearm  200 . Accordingly, reducing this type of frictional force provides more efficient operation. 
     As best shown in  FIGS. 9 and 11B , one embodiment of recoil assembly  20  comprises two recoil springs  53  extending in a spaced-apart generally parallel relationship. Each recoil spring  53  is installed concentrically along a recoil guide rod  54  and maintained in position between forward spring retainer  48  and aft spring retainer  49 . In some embodiments, recoil springs  53  and guide rods  54  extend along opposite sides of recoil interface body  67  that includes aft spring retainer  49 , pivoting interface  61 , and tabs  141 . As shown in  FIG. 10 , a set of springs may be included in recoil assembly  20 . The set of springs may include a primary spring or plunger return spring  47  (shown in  FIG. 12 ) acting on return plunger  51  and a secondary spring or recoil spring  53  acting on op-rod  50  between forward spring retainer  48  and aft spring retainer  49 . 
     As illustrated herein, recoil assembly  20  includes pivoting interface  61  as discussed above to allow upward rotation of op-rod  50  along a median plane  7  relative to carrier assembly  40 . The rotation of recoil assembly  20  with respect to carrier  38  is minimized once recoil assembly  20  is inserted into a firearm  200 . When installed in firearm  200 , pivoting interface  61  serves primarily to absorb moment forces resulting from firing and translate carrier assembly  40  axially along bore axis  5  with little or no rotational deviation from bore axis  5 . 
     Firing Pin Lock 
     Another aspect of the present disclosure is directed to a firing pin lock  109  with a firing pin catch  59 .  FIG. 12  illustrates a left side sectional view showing bolt assembly  30  with firing pin  41 , carrier assembly  40 , and components of recoil assembly  20  in accordance with an embodiment of the present disclosure.  FIG. 13  illustrates a side elevational view of firing pin  41  in accordance with an embodiment of the present disclosure.  FIGS. 14A-14C  illustrate the position of hammer  44 , firing pin lever  105 , and firing pin catch  59  at various points during the firing sequence. 
     In one embodiment, firing pin lever  105  is a component of recoil assembly  20  and includes firing pin catch  59 . For example, firing pin lever  105  can be built into or pivotably attached to op-rod  50 . In one embodiment, firing pin lever  105  is pivotably attached to recoil assembly  20  and includes firing pin catch  59  extending therefrom. Firing pin catch  59  is configured to prevent the firing pin  41  from inadvertently moving forward and hitting the primer. When recoil assembly  20  is installed in carrier assembly  40 , firing pin catch  59  extends to engage firing pin  41  and prevents firing pin  41  from moving forward. Accordingly, firing pin catch  59  prevents firing pin  41  from inadvertently hitting the primer during normal operation and when firearm  200  is dropped. 
     Referring to  FIG. 13 , a side illustration shows firing pin  41  including a camming mechanism  130  at the head  110  of firing pin  41  in accordance with an embodiment of the present disclosure. Firing pin  41  includes a head  110  on an aft end. Head  110  is adjacent a narrowed region  112  that defines a catch surface  114  at a forward face of head  110 . Firing pin catch  59  engages catch surface  114  of firing pin  41  to prevent forward motion until the trigger is pulled. 
       FIG. 12  illustrates an internal configuration of the firing pin lock  109  in a resting state. Firing pin catch  59  occupies the space defined by narrowed region  112  of firing pin  41  and abuts catch surface  114 . Thus, firing pin catch  59  blocks firing pin  41  from forward movement.  FIGS. 14A-14C  illustrate a stepwise progression of component movement during firing. 
       FIG. 14A  illustrates the fire control group  145  after pulling the trigger, where hammer  44  is rotating forward and about to strike head  110  of firing pin  41 . When the trigger is pulled, hammer  44  rotates forward toward firing pin  41 , an end portion  44   a  of hammer  44  contacts firing pin lever  105 , thereby starting to pivot firing pin catch  59 .  FIG. 14B  illustrates firing pin lever  105  deflected upwards as hammer  44  contacts head  110  of firing pin  41 . As shown in  FIG. 14C , firing pin lever  105  and firing pin catch  59  have been deflected upward out of engagement with firing pin  41 , which is then no longer obstructed from moving forward to strike the primer and discharge firearm  200 . 
     After firing firearm  200 , firing pin return spring  43  (shown in  FIG. 12 ) causes the firing pin  41  to reset toward the rear, thereby allowing the firing pin catch  59  to occupy narrowed region  112  of firing pin  41  where it is positioned to prevent forward motion of firing pin  41  until the trigger is pulled again. Rearward motion of firing pin  41  is stopped by firing pin retention pin  58  extending into carrier  38  (shown in  FIG. 3 ). Also after firing, hammer  44  pivots down to return to the cocked position and providing space for firing pin lock  109  to move down and back into position to hold back firing pin  41 . 
     In some embodiments, firing pin return spring  43  may be included to bias firing pin  41  aft toward a resting state. Firing pin return spring  43 , shown in  FIG. 12 , ensures that firing pin  41  is positioned aft and secured behind firing pin catch  59  prior to firing. Alternatively, in some embodiments, a camming mechanism may be used to bias firing pin  41  toward a resting state. For example, head  110  of firing pin  41  (shown in  FIG. 13 ) may have a forward portion formed as a camming surface  130  to bias firing pin  41  toward a resting state and an aft portion formed as a stop surface  120 . In a resting state, stop surface  120  is positioned against a bottom surface of firing pin catch  59 . Following release of a round, camming surface  130  biases firing pin  41  back into a resting state. 
     Hollowed-Out Piston 
     In some cases, it may be desirable to increase the volume in the gas system, particularly in firearms that are equipped with suppressors. Referring to  FIGS. 15 and 16 , which show a side elevational view and cross-sectional view, respectively, an example of a gas system  12  in accordance with an embodiment of the present disclosure is illustrated. In one aspect, a hollowed-out piston  13  is provided with firearm  200 . A gas block  14  and gas manifold  16  direct propellant gases from barrel  11  into piston  13 . The hollowed-out piston  13  is configured to maximize volume in the gas system  12 , which in turn requires a longer stroke to operate the weapon. This results in lower loads and delays moving the bolt to the rearward position to open the chamber. The delayed opening of the chamber provides more time for propellant gases to expand through the suppressor  15  and therefore improves suppressor performance. The hollowed-out piston  13  can also provide more space for a gas valve  16 . Gas can move from barrel  11  into the space created in hollowed-out cavity  18  in piston  13 . The larger available volume allows for more time to reach a given pressure; this delay allows gas to bleed out of barrel  11 . Embodiments described herein provide the advantages of a slower movement of gas through the gas system which does not accelerate parts faster than may be desired. A hollowed-out piston  13  can be, for example, a cylinder that is closed at one end and that receives the op-rod  50  through an opening in an opposite end. Hollowed-out cavity  18  may have a volume greater than 10%, 20%, 30%, 40%, 50%, or 60% or more of the volume of the piston. Propellant gases are directed to expand into the hollowed-out cylinder  13  and provide pressure sufficient to move op-rod  50  rearward to open the chamber. 
       FIG. 17  illustrates an example firearm  200  that could be configured with the operating system described herein. Firearm  200  includes lower receiver  75  housing the fire control group  145  and an upper receiver  76  housing recoil assembly  20  and the bolt carrier group that includes bolt assembly  30  and carrier assembly  30 . 
     The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
     The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims.