Abstract:
Implementations of the present invention relate to apparatuses, systems, and methods for converting an M249-platform firearm to allow the use of 6.8 mm SPC ammunition therein. The conversion kit comprises an adapted barrel, barrel extension, gas port, and bolt, which are each configured to allow rapid reconfiguration of the firearm between calibers.

Description:
PRIORITY 
       [0001]    The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/926,755 entitled “6.8 MM SPC CONVERSION KIT” filed Jan. 13, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0002]    1. The Field of the Invention 
         [0003]    Generally, this disclosure relates to firearms. More specifically, the present disclosure relates to methods, devices, and systems for providing a DOD-designation M249-type platform firearm capable of firing 6.8 mm SPC I and II ammunition. 
         [0004]    2. Background and Relevant Art 
         [0005]    Belt-fed machine guns generally fall into two broad categories based on the way the gun fires ammunition: open-bolt or closed-bolt. In an open-bolt gun, the operating group, which includes the bolt, is held toward the rear of the receiver and away from chamber when not firing. The operating group is restrained, under tension from a spring, such that when the operating group is released, it moves forward forcefully. The forward movement shears a bullet off of a belt, delivers the bullet to the chamber, closed the chamber, and fires the bullet. In a closed-bolt gun, the operating group is held forward and against the barrel extension when not firing. The bolt is mated and locked to the barrel extension forming a closed chamber. The chamber may house a bullet waiting to be fired by an impulse from a hammer or other impulse source delivered to the bullet&#39;s primer by a firing pin. 
         [0006]    An open-bolt gun is inherently a machine gun. Without input from an operator, an open-bolt gun will continuously fire, typically at a very high rate, as long as the weapon has ammunition or until the gun malfunctions. Each time the operating group moves forward in an open-bolt gun, the forward motion detonates the bullet&#39;s primer, firing the gun. The firing of a bullet generates a rapidly expanding gas within the barrel and some of the gas is diverted to a gas piston which forces the operating group rearward, opening the chamber and moving the next round into position, before a spring forces the operating group forward again, repeating the process until the ammunition is exhausted or an operator restrains the operating group in a rearward position. 
         [0007]    A closed-bolt gun, conversely, may remain at rest with the operating group forward and a bullet chambered. The firing pin remains withdrawn from the bullet until an impulse source, such as a hammer or a striker, delivers an impulse to the firing pin to detonate the primer and charge in the bullet. At which time, the expanding gas in the barrel may be diverted to provide energy to cycle the operating group similarly to an open-bolt gun, except when the spring returns the operating group to a forward position, the bolt locks adjacent the barrel extension and the bullet in the chamber awaits the operator releasing the impulse source. 
         [0008]    Prior to the Firearms Owners&#39; Protection Act of 1986, open-bolt machine guns could be newly registered legally in the United States. The FABRIQUE NATIONALE D′HERSTAL (“FN”) MINIMI open-bolt machine gun (and the affiliated United States variant, the M249 light machine gun platform) was among the most common open-bolt machine guns available at the time, and remains one of the most common open-bolt machine guns in the world. The FN MINIMI was originally developed in 1974 and has continued in operation with militaries in 45 countries. There are a great deal of parts, accessories, and assemblies available for the platform on the market, and the transfer of open-bolt machine guns legally registered before May 19, 1986 is legal through proper channels and with proper documentation. However, the production of new open-bolt machine guns, such as the M249 platform, for civilian sale in the United States is now illegal. Due to the reputation and restricted availability of the M249 platform, there remains a demand for M249-type firearms among civilians, as well as a robust market around the original guns. 
         [0009]    However, an open-bolt belt-fed machine gun, such as the M249 platform has a number of disadvantages for use in military or law enforcement conflicts despite the high rate of fire of the weapon. Typically, the high rate of fire of the M249 platform (approximately 800 rounds per minute) results in challenges for the operator to control the recoil and therefore accuracy of the weapon. Furthermore, in many cases, the advantages of outputting up to 800 rounds per minute may be outweighed by the consumption of ammunition. For example, 200 rounds of 5.56 mm×45 mm NATO ammunition, not including the belt links, weighs almost 6 pounds and an M249-platform machine gun can fire all 6 pounds of ammunition in 15 seconds. The M249 platform also supports a 7.62 mm×51 mm NATO variant that weighs twice as much per round. Therefore, mobility of the gun and operator is directly tied to ammunition consumption and ammunition type. 
         [0010]    Additionally, belt-fed self-loading rifles, whether open-bolt machine guns or closed-bolt rifles capable of a number of firing modes, are utilized around the world by a variety of entities, in a variety of environments, for a variety of applications. Depending on these factors and others, the desired ammunition to be used with the firearm may change. For applications requiring heavier ammunition in order to combat armored or otherwise protected targets, a larger caliber ammunition or higher energy ammunition may be desired. For applications requiring greater accuracy or greater ammunition capacity (meaning more rounds of ammunition may be carried) an operator may prefer to use a smaller caliber bullet with lower energy charge in the ammunition. Furthermore, a single entity, such as a national military, may find it beneficial to utilize a single type of ammunition in as many of its weapons as possible to promote interoperability between its ammunition supply and its stock of firearms. 6.8 mm SPC ammunition (and derivatives, such as 6.8 mm SPC-2 ammunition) offers a caliber and energy between that of the popular 5.56 mm×45 mm NATO ammunition and the 7.62 mm×51 mm NATO ammunition. 
         [0011]    Therefore, it is desirable to allow firearms of the M249 platform to fire 6.8 mm SPC ammunition. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0012]    Implementations of the present disclosure solve one or more of the foregoing or other problems in the art with apparatuses, systems, and methods for firing 6.8 mm SPC ammunition from an M249 platform firearm. A firearm capable of firing 6.8 mm SPC ammunition may comprise a barrel having a barrel bore sufficient to allow passage of a standard 6.8 mm SPC round while enabling rifling on the interior surface of the bore and a chamber sufficient to hold a standard 6.8 mm SPC round in chamber. The firearm may also comprise a bolt having a bolt face of sufficient diameter to reliably align with and securely retain a standard 6.8 mm SPC round during firing. The firearm may also comprise a gas port disposed laterally in the barrel, the gas port having a diameter appropriate to provide sufficient gas pressure after firing to cycle an operating group without damaging the gun. 
         [0013]    The present disclosure also relates to the modification or replacement of the bolt, barrel, and/or barrel extension to allow an M249-type firearm to fire standard 6.8 mm REMINGTON Special Purpose Cartridge (hereinafter “SPC”) ammunition. A closed-bolt variant of the M249 platform may include other modifications such as the carrier, slide, recoil spring, gas tube, trunnion, gas block, grip, trigger housing, and operating rod; and a sear and trigger of the open-bolt system may be replaced with trigger package containing a hammer or other impulse source. Any description of a closed-bolt variant should be understood to be merely illustrative and not be understood to exclude the more common open-bolt M249 platform firearms. 
         [0014]    Additional features and advantages of exemplary implementations of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of such exemplary implementations. The features and advantages of such implementations may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features will become more fully apparent from the following description and appended claims, or may be learned by the practice of such exemplary implementations as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. For better understanding, the like elements have been designated by like reference numbers throughout the various accompanying figures. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0016]      FIG. 1  is an isometric exploded view of a firearm according to the present disclosure; 
           [0017]      FIG. 2  is a lower isometric exploded view of the firearm of  FIG. 1 ; 
           [0018]      FIG. 3  is an isometric view of an integrated slide-carrier according to the present disclosure; 
           [0019]      FIG. 4  is a left side view of the integrated slide-carrier of  FIG. 3 ; 
           [0020]      FIG. 5  is a left side cross-sectional view of the integrated slide-carrier of  FIG. 3 ; 
           [0021]      FIG. 6  is a left side cross-sectional view of the integrated slide-carrier of  FIG. 3 , further including a firing pin and firing block; 
           [0022]      FIG. 7  is a rear end view of the integrated slide-carrier and firing block of  FIG. 6 ; 
           [0023]      FIG. 8  is a front end view of the integrated slide-carrier of  FIG. 3 ; 
           [0024]      FIG. 9  is an isometric view of the firing block of  FIG. 6 ; 
           [0025]      FIGS. 10A-C  are left side views of the rotation of a bolt due to linear movement of the integrated slide-carrier of  FIG. 3 ; 
           [0026]      FIGS. 11A-C  are left side cross-sectional views of the rotation of a bolt due to linear movement of the integrated slide-carrier of  FIG. 3 ; 
           [0027]      FIGS. 12A-B  are left side cross-sectional views the detonation of a bullet by transmitting an impulse through the firing block of  FIG. 6 ; 
           [0028]      FIGS. 13A-C  are left side cross-sectional views of resetting a hammer due to the linear movement of the integrated slide-carrier of  FIG. 3 ; 
           [0029]      FIGS. 14A-C  depict the use of a selector stop with a fire mode selector switch; 
           [0030]      FIG. 15  is an exploded view of the removable trigger package and selector switch; 
           [0031]      FIG. 16  is a left side cross-sectional of caliber conversion kit; 
           [0032]      FIG. 17  is a front end view of a bolt according to the present disclosure; 
           [0033]      FIG. 18  is a left side cross-sectional view of the bolt of  FIG. 17 ; 
           [0034]      FIG. 19  is a left side cross-sectional detail view of the barrel and chamber-forming elements of  FIG. 16 ; and 
           [0035]      FIG. 20  is a left side cross-sectional detail view of the gas port and gas block of  FIG. 19 ; 
           [0036]      FIG. 21  is a perspective view of a feed tray that may be used in conjunction with the conversion kit of  FIG. 16 ; and 
           [0037]      FIG. 22  is a perspective exploded view of an open-bolt machine gun which may be converted using the conversion kit of  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The FABRIQUE NATIONALE D′HERSTAL (“FN”) MINIMI platform is one of the most common light machine gun platforms in the world, including many variants and having countless available accessories. Subsequent variants of the FN MINIMI include the DOD-designation M249, MK46, MK48, and the MGA SAW. As used herein, “M249 platform” should be understood to encompass any firearm derived from the FN MINIMI design including, but not limited to, the M249 firearm. The M249 platform is an open-bolt, slam fire weapon, but some variants may be a closed-bolt, semi-automatic variant. The present disclosure contemplates the conversion of any M249 platform to fire 6.8 mm SPC I and II ammunition. 
         [0039]    The present disclosure also relates to the modification or replacement of the bolt, barrel, and/or barrel extension to allow an M249-type firearm to fire standard 6.8 mm REMINGTON Special Purpose Cartridge (hereinafter “SPC”) ammunition. A closed-bolt variant of the M249 platform may include other modifications such as the carrier, slide, recoil spring, gas tube, trunnion, gas block, grip, trigger housing, and operating rod; and a sear and trigger of the open-bolt system may be replaced with trigger package containing a hammer or other impulse source. Any description of a closed-bolt variant should be understood to be merely illustrative and not be understood to exclude the more common open-bolt M249 platform firearms. 
         [0040]    A closed-bolt operating group may include an integrate slide-carrier that enables the use of a substantially standard bolt, firing pin, and trigger package, while translating the force applied from a first axis to a second axis in order to allow proper operation of the firearm in a semi-automatic, burst-fire, or fully-automatic firing mode. The first and second axes may each be longitudinal axis and, therefore, parallel or non-parallel axes, such as perpendicular or at an acute angle to one another. Furthermore, the directions of the forces, even when the axes are parallel, may not be the same. 
         [0041]    The integrated slide-carrier may incorporate the functionality of a slide and carrier while allowing additional functionality by removing the division and, hence, connection therebetween. The slide-carrier may allow for more reliable operation of the gun with less moving parts to replace or maintain and for less chance of failure in the field. The slide-carrier may also allow the transmission of a firing force from an impulse source through the slide-carrier to a firing pin, which may then transmit the force to a propellant in the ammunition. The slide-carrier may also enable the translation of the firing force in a non-linear path or along more than one axis. 
         [0042]    The elimination of the connection between the slide and carrier may enable the integrated slide-carrier to transmit force from expanding gas rod to the slide more directly. The monolithic construction of the integrated slide-carrier may thereby reduce torque applied on receiver rails to which the slide-carrier is slidably mounted. Reduced torque on the slide may reduce wear on the receiver rails, providing a further increase in reliability and reduction in maintenance of the firearm. 
         [0043]      FIG. 1  depicts an isometric exploded view of the main operational components of an embodiment of a firearm  100  including an integrated slide-carrier assembly.  FIG. 2  depicts a lower isometric exploded view of the main components of the firearm  100 . The firearm  100  includes a receiver  200 , which may carry upon it various information engraved or otherwise affixed thereto. The information on the receiver  200  may commonly include model designation and identification information unique to that receiver to identify the firearm  100  for registration and ownership purposes. The receiver  200  may also enable the connection and assembly of many of the operational components on or in the receiver  200 . For example, the receiver  200  includes a receiver body  202  that defines an interior channel  204  with left and right receiver rails  206   a ,  206   b  affixed thereto. The left receiver rail  206   a  and right receiver rail  206   b  may be symmetrical with respect to one another, or they may be asymmetrical. For example, the left receiver rail  206   a  and the right receiver rail  206   b  may have differing thicknesses or they may be positioned differently in the interior channel  204 . The left receiver rail  206   a  may be thicker or thinner than the right receiver rail  206   b . Additionally or alternatively, the left receiver rail  206   a  may be positioned higher or lower than the right receiver rail  206   b . Furthermore, the left receiver rail  206   a  may be longer or shorter longitudinally within the interior channel  204  than the right receiver rail  206   b . The receiver  200  further comprises a selector stop  210 . The selector stop  210  may be affixed to an exterior surface of the receiver or may be a raised portion of the receiver itself. The selector stop  210  inhibits a fire mode selector switch  512  such as that found on commercially available hammer-operated trigger packages from reaching a “disassemble” position, as will be explain in relation to  FIGS. 14A-C . 
         [0044]    The operating group  300  is slidably connected to the receiver  200  by the left and right receiver rails  206   a ,  206   b . The operating group  300  includes the integrated slide-carrier  302  (described further in  FIGS. 3-8 ) having an elongate upper section in which there are left and right longitudinal recessions  304   a ,  304   b . The left and right longitudinal recessions  304   a ,  304   b  receive the left and right receiver rails  206   a ,  206   b , respectively, to allow the longitudinal movement of the operating group  300  within the interior channel  204  of the receiver  200 . The operating group  300  further includes a firing block  306  that is disposed at least partially inside the integrated slide-carrier  302 . Alternatively, the firing block  306  may be disposed entirely externally to the integrated slide-carrier. (The firing block  306  will also be described more fully in relation to  FIGS. 5-9 .) The firing block  306  transmits a force to the firing pin assembly  308 , which is at least partially disposed within a bolt  310 . The bolt  310  includes notches, grooves, channels, or threads for selectively connecting to another, complementary connector. 
         [0045]    Still referring to  FIG. 1 , the receiver  200  also includes a central trunnion  208  into which the barrel assembly  400  connects. The barrel assembly  400  comprises a barrel body  402  that includes a bore  404  therethrough. The bore  404  provides communication between the barrel body  402  and a barrel extension  406 . Together, the barrel extension  406  and the bore  404  provide a path through which a bullet (not shown) may exit the firearm  100 . 
         [0046]    The barrel assembly  400  also includes a gas block  408  disposed on the barrel body  402  forward of the barrel extension  406 . The gas block  408  covers a gas port  410  and provides fluid communication with a gas block outlet  412 . After firing a bullet, rapidly expanding gas may travel the length of the barrel body  402  through the bore  404 . As the gas passes the gas port  410 , the gas block  408  may channel some of the gas laterally away from the bore  404  and toward the gas block outlet  412 . The diverted gas may be expelled through the gas block outlet  412  and provide the motive force to cycle the firearm  100  and prepare for a subsequent firing. 
         [0047]    The barrel assembly  400  connects to the receiver  200  by inserting the barrel extension  406  into the central trunnion  208 . The barrel extension  406  may connect to the trunnion  208  via threads, a twist lock, a friction fit, a weld, an adhesive or other secure attachment. The connection between the barrel  406  and the trunnion  208  may be selectively attachable to facilitate maintenance and repair of the firearm  100 . The barrel extension  406  provides complementary notches, grooves, channels, or threads into which the bolt  310  may be received and selectively secured thereto. The connection of the bolt  310  to the barrel extension  406  provides a selectively securable connection between the barrel assembly  400  and the internal operating group  300 . The connection of the operating group  300  and the barrel assembly  400  provides a chamber in which a bullet may be held and fired (visible in  FIGS. 12A-B ). 
         [0048]    Still referring to  FIG. 1 , the firearm  100  further includes a control assembly  500  disposed on the underside of the firearm  100  and selectively connected to the receiver  200 . The control assembly includes a housing  502  with front mounting points  504  and rear mounting points  506 . The front mounting points  504  may be a notch that is configured to be received into a recession on the receiver body  202 , eyelets for a cross-bar, a snap fit, or other similar selectively securable connection. Similarly, the rear mounting points  506  may be a notch configured to be received into a recession on the receiver body  202 , eyelets for a cross-bar, a snap fit, or other similar selectively securable connection. A trigger package  508  is disposed within the housing  502  of the control assembly  500 . The trigger package includes an impulse source such as a hammer  510 , as depicted in  FIG. 1 , or a striker or other similar linear actuator. The trigger package  508  may be a commercially available trigger package and may include safe, semi-automatic, 2-round burst, 3-round burst, fully automatic, or other fire operation modes selectable with a fire mode selector switch  512 . The trigger package  508 , more specifically, may comprise a HECKLER AND KOCH trigger package. The trigger package  508  may operate the firearm  100  without modification to the trigger mechanism. Other modifications not affecting the trigger mechanism may include, for example, removal of the ejector. 
         [0049]    Continuing to refer to  FIG. 1 , the firearm  100  further comprises a gas piston assembly  600  that provides a fluid and mechanical linkage between the barrel assembly  400  and the operating group  300 . The gas piston assembly  600  connects the barrel assembly  400  to the operating group  300  by a gas piston-and-cylinder linkage. The gas tube  602  is disposed around, or otherwise forms a fluid seal with, the gas block outlet  412 . The gas block outlet  412  may provide a source of high pressure gas, which may impinge upon a surface of a gas piston  604 . The gas piston  604  is connected to a rigid operating rod  606 , which is, in turn, connected to the operating group  300 . The operating rod  606  is connected to the operating rod connection  312  on the integrated slide-carrier  302  of the operating group  300 . The connection between the operating rod  606  and the operating rod connection  312 , and the connection between the gas piston  604  and the operating rod  606 , may be any connection of sufficient strength to communicate the compressive and tensile forces produced during operation of the firearm  100 . For example, the connection may be threads, a twist lock, a friction fit, a weld, an adhesive or other secure attachment. Preferably the connection may be a selective connection facilitating maintenance and repair of the firearm  100 , and more preferably, the connection may be adjustable to allow precise tuning of the operation of the firearm  100 . For example, the connection may be a threaded connection providing a selective and adjustable connection. A threaded connection may further comprise a lateral set screw to retain the connection at the selected relative position. 
         [0050]    The gas piston assembly  600  may allow the high pressure gas, the gas contained within the barrel bore  404  and directed through the gas block  408  and gas port  410  to the gas block outlet  412 , to provide the energy for a motive force to cycle the operating group  300 . The motive force may be a reciprocal linear force resulting from the pressure of the impinging gas from the gas block outlet  412  in the rearward direction, and an opposite linear force from a recoil spring  608  disposed circumferentially around the operating rod and compressed between a surface of the gas piston  604  and a bushing  610  disposed adjacent the trunnion  208 . The bushing  610  is an annular bushing configured to allow the operating rod  606  to slide through a central opening in the bushing  610  while the recoil spring  608  is retained by an annular surface of the bushing  610 . Hence, when the high pressure gas impinges upon the gas piston  604 , the gas piston  604  travels rearward along the length of the gas tube  602 , and compresses the recoil spring  608  against the bushing  610  adjacent the trunnion  208 . The seal between the gas piston  604  and the gas tube  602  allows for the passage of a portion of the high pressure gas, allowing dissipation of the pressure in the gas tube  602 . The gas that escapes beyond the gas piston  604  may then pass through channels in the bushing  610  and escape the firearm  100 , dissipating the gas in the gas tube  602 . 
         [0051]    The recoil spring  608  may then provide a restoring force in opposition to the rearward movement of the gas piston  604 . The restoring force causes the gas piston  604  to travel forward in the gas tube  602  until the gas piston  604  returns to a position adjacent the gas block outlet  412 . Thus, each firing of the firearm  100  may result in a reciprocal motion of the gas piston  604  within the gas tube  602 . The reciprocal motion of the gas piston  604  within the gas tube  602  with each firing of the firearm  100  provides the motive force to reciprocally move the operating group  300  within the receiver  200 . 
         [0052]    The reciprocal motion of the operating group  300  may provide the input force for nearly all other operations of the firearm  100 , as will be discussed in relation to  FIGS. 10-15 . For example, the motion of the operating group  300  after the firing of a first round and the introduction of high-pressure gas through the gas port  610  and into the gas tube  602 , unlocks the bolt  310  from the barrel extension  406 , extracts a shell casing, ejects the shell casing, resets the trigger package  508 , removes a second round from an ammunition source, inserts the second round into the barrel extension  406 , and then locks the bolt  310  in the barrel extension  406 . Many of these functions are provided by the integrated slide-carrier  302  of the operating group  300 , depicted in detail in  FIGS. 3-8 . 
         [0053]    As can also be seen in  FIG. 1 , the firearm  100  comprises a top cover  700 , as is known in the art, configured to feed in a belt of ammunition. The top cover  700  feeds ammunition with a lever-activated feed driven by the bearing  328  of the operating group  300 . The bearing  328  may follow a track in the top cover  700  providing an incremental, lateral feed of ammunition, as is visible in  FIG. 2 . The top cover  700  is specific to the type and size of ammunition being fired. 
         [0054]    Referring now to  FIG. 3 , the integrated slide-carrier  302  comprises the left and right longitudinal recessions  304   a ,  304   b , which receive the left and right receiver rails  206   a ,  206   b  respectively to facilitate the longitudinal, reciprocal movement of the operating group  300  within the interior channel  204  of the receiver  200 . The integrated slide-carrier  302  also comprises a slide bore  314 , into which a firing pin  308  and bolt  310  (not depicted) may be inserted. The bore extends from near a forward end of the integrated slide-carrier  302  substantially through the length of the integrated slide-carrier  302 , but not through the entire integrated slide-carrier  302 . The bore is recessed from a front end of the integrated slide-carrier  302  to allow the bolt  310  (not depicted) to properly lock into the barrel extension  406 . 
         [0055]    Referring now to  FIG. 4 , the front end of the integrated slide-carrier  302  comprises an upper front surface  316   a  and a lower front surface  316   b , which are co-planar. The co-planar upper front surface  316   a  and lower front surface  316   b  extend on either side of the barrel extension  406  when the firearm  100  is in battery. The integrated slide-carrier  302  is held against the barrel extension  406  by the recoil spring  608  and the operating rod  606  connected to the operating rod connector  312 . A contact surface  316   c  may distribute the compressive force between the integrated slide-carrier  302  and the barrel extension  406  to reduce strain and wear on the integrated slide carrier  302 . 
         [0056]    Still referring to  FIG. 4 , the integrated slide-carrier  302  further comprises a rotation channel  318  associated with the slide bore  314 . The rotation channel  318  guides the rotation of the bolt  310  to lock and unlock the bolt  310  from the complementary channels in the barrel extension  406 . The rotation channel  318  comprises an upper portion  318   a , a catch  318   b , a rotational portion  318   c , and a longitudinal portion  318   d . The upper portion  318   a  has a rearward slanted front face and a vertical rear face, while the rotational portion  318   c  has a forward slanted front face and forward slanted rear face, while the catch  318   b  forms the junction of the upper portion  318   a  and the rotational portion  318   c . The upper portion  318   a  allows manual removal or installation of a bolt  310  by rotating the bolt  310  through the upper portion  318   a  and drawing the bolt  310  out through the slide bore  314 . During normal operation, however, the catch  318   b  prevents the unintended removal of the bolt  310 . 
         [0057]    Still referring to  FIG. 4 , the integrate slide-carrier  302  comprises a lower support  320 . The lower support  320  provides structural support to the integrated slide-carrier  302  and thereby reduces strain and wear on the integrated slide-carrier  302  to prevent failure of the operating group  300 . The lower support  320  extends substantially the length of the integrated slide-carrier  302  and defines a central space  322 . The lower support  320  connects to the remainder of the integrated slide-carrier  302  by one or more points. The central space  322  is devoid of material or may comprise material of different mass than the integrated slide-carrier  302 , in order to tune the mass of the operating group  300 . The mass of the operating group  300  may need to change to ensure proper operation of the firearm  100  depending on operating conditions, ammunition type, the spring constant of the recoil spring  608 , the size of the gas port  410 , or other factors. 
         [0058]    The integrated slide-carrier  302  additionally comprises a sear release arm  324 , enabling the firearm  100  to be operated in a fully automatic firing mode. The sear release arm  324  is configured to release a sear in a hammer-operated fully automatic firing mechanism, such as some HECKLER AND KOCH trigger packages. The integrated slide-carrier  302  also comprises a bevel  326  configured to engage a hammer  510  or other impulse source of a trigger package  508  and reset the hammer  510  or other impulse source as the operating group  300  cycles rearward after firing. The integrated slide-carrier  302  may also comprise a channel configured to hold the bearing  328  which may engage with a top cover  700  (not depicted) to feed ammunition automatically into the firearm  100 . 
         [0059]    As shown in  FIG. 5 , the slide bore  314  extends through some, but not all of the integrated slide-carrier  302 . Alternatively, the slide bore  314  may extend through substantially the entire length of the integrated slide-carrier  302 . The slide bore  314  includes a hole for a bore cross-pin  330  that intersects the slide bore  314  and may retain the firing pin  308  within the slide bore  314 . The bore cross-pin  330  retains the firing pin  308  within a desired range of motion, allowing for the selective extension of the firing pin  308  through and out of the bolt  310  to set off the ammunition when in battery. 
         [0060]    The integrated slide-carrier  302  includes a rear channel  334 , which communicates with the slide bore  314  in a rear portion of the slide bore  314 . The rear channel  334  of the integrated slide-carrier  302  includes rear channel rails  336  recessed into the sides of the rear channel  334 . The rear channel rails  336  extend forward from a rear surface of the integrated slide-carrier  302  and may be symmetrical on opposing faces of the rear channel  334 . As can be seen in  FIG. 6-8 , the firing block  306  is disposed at least partially within the rear channel  334 , at least partially within the slide bore  314 , and at least partially outside of the integrated slide-carrier  302 . Alternatively, the firing block  306  may be disposed externally to the integrated slide-carrier  302 . 
         [0061]    As shown in  FIG. 7 , the firing block  306  is disposed between the substantially opposing lateral faces of the rear channel  334  and substantially fills a lateral width of the rear channel  334 . The width of the firing block  306  is such that the firing block  306  cannot turn laterally and jam within the rear channel  334 . The firing block  306  comprises firing block rails  338  that align with the rear channel rails  336  disposed in the lateral faces of the rear channel  334 . The rear channel rails  336  and the firing block rails  338  may be identical but mirrored versions of one another, but need not be. For example, the rear channel rails  336  and the firing block rails  338  of  FIG. 7  are both semi-circular in transverse cross-section, but in other embodiments may be triangular in transverse cross-section, or may be rectangular in transverse cross-section. Alternatively, the rear channel rails  336  may be semi-circular in transverse cross-section, triangular in transverse cross-section, or rectangular in transverse cross-section, and the firing block rails  338  may have a different cross-section. 
         [0062]    In any configuration, the rear channel rails  336  and the firing block rails  338  may form a cavity in which a guide pin  340  (shown in dashed lines in  FIG. 7 ) may be disposed.  FIG. 7  depicts an integrated slide-carrier  302  and firing block  306  with two pairs of rear channel rails  336  and firing block rails  338  providing two cavities in which two guide pins  340  are disposed. The guide pins  340  retain the firing block  306  along a longitudinal path of travel and restrict the longitudinal rotation of the firing block  306  such that the firing block does not jam in the rear channel  334  or the slide bore  314  during longitudinal movement. The guide pins  340  are retained by a rail cross-pin  332  that inhibits rearward movement of the guide pins  340 . 
         [0063]    As shown in  FIG. 8 , the rear channel  334  intersects with the slide bore  314 , but the slide bore  314  and the rear channel  334  only partially overlap due to the slide bore  314  extending only part of the length of the integrated slide-carrier  302  and not extending all the way to the rear of the integrated slide-carrier  302 . The firing block  306  is, therefore inserted into the rearward portion of the slide bore  314  and then held within a predetermined range of positions by the guide pins  340 . 
         [0064]      FIG. 9  depicts the firing block  306  that is disposed at least partially within the rear channel  334 , at least partially within the slide bore  314 , and at least partially outside of the integrated slide-carrier  302 . The firing block  306  transfers energy from a hammer  510  or other impulse source in a trigger package  508  on a first axis to a firing pin  308  on a longitudinal second axis. The first axis is also longitudinal, but need not be in alternative embodiments. Similarly, the second axis is parallel to the first axis, but need not be in alternative embodiments. The firing block  306  is generally L-shaped, but in other embodiments, the firing block may be triangular, rectangular, or any other shape capable of transferring mechanical forces from a first axis to a second, parallel axis. The firing block  306  comprises a firing pin contact surface  342  and a hammer contact surface  344 . The firing pin contact surface  342  is configured to deliver an impulse to the firing pin  308  reliably, and therefore includes a flat surface to be disposed in contact with, or adjacent to a rearward end of the firing pin  308 . The firing pin contact surface  342  protrudes forward into the slide bore  314  and beyond the rear channel  334 . The firing pin contact surface  342  protruding beyond the rear channel  334  allows the firing pin contact surface  342  to contact the rear end of the firing pin  308  without needing the rear end of the firing pin  308  to extend past the forward end of the rear channel  334 . If the firing pin  308  extends too far rearward, the firing pin  308  may catch on the forward end of the rear channel  334  and could lead to the firearm  100  jamming during operation. 
         [0065]    The hammer contact surface  344  disposed is at the rear of the firing block  306  and extends beyond the rear end of the integrated slide-carrier  302  such that a hammer or other impulse source from the trigger package  504  may contact the hammer contact surface  344 . The hammer contact surface  344  is configured to receive an impulse from the trigger package  508  reliably, and therefore includes a flat surface to be disposed in contact with, or adjacent to, a hammer  510  or other impulse source of the trigger package  508 . Additionally, to withstand the receipt of and to properly transmit tens or hundreds of thousands of impulses from the trigger package  508 , the firing block  306  is reinforced in some areas and lightened in other areas. For example, the firing block  306  may have additional material in a flared portion  346  leading to the hammer contact surface  344 . The additional material in the flared portion  346  toughens the firing block  306  in that region and enhances the operational lifetime of the firing block  306 . 
         [0066]    Furthermore, the firing block  306  comprises a brace  348  that extends diagonally from the corner of the generally L-shaped firing block  306 . The brace  348  aids in transmitting the impulse from the trigger package  508  to the firing pin  308  sufficiently efficiently to allow the removal of material elsewhere, such as a void  350 , without degrading the performance of the firing block  306 . By removing material and having a void  350  in the firing block  306 , the overall mass and therefore inertia of firing block  306  may be reduced, resulting in a more immediate transfer of energy from the trigger package  508  to the firing pin  308 . Also, a firing block  306  of greater mass and inertia may be more likely to prematurely firing the firearm  100  when the operating group  300  cycles forward. To ensure the firing block  306  remains within the desired range of movement, a pin slot  352  is included near the hammer contact surface  344  through which the rail cross-pin  332  is disposed, restricting movement of the firing block  306  and ensuring the firing block does not fall out of the integrated slide-carrier  302 . 
         [0067]    Referring now to  FIG. 10A-C , the catch  318   b  retains the bolt  310  and urges the bolt  310  rearward during rearward motion of the integrated slide-carrier  302  and assists in aligning the bolt head  310   a  with the barrel extension  406  (barrel extension  406  not depicted in  FIGS. 10A-C ). Upon forward motion of the operating group  300  toward the barrel extension  406 , the bolt  310  contacts the barrel extension  406  first and the integrated slide-carrier  302  continues moving forward, compressing a firing pin spring  354  and pushing the bolt  310  into the slide bore  314 . The firing pin spring  354  is at least partially recessed into an annular recession in the bolt  310  to prevent kinking of the firing pin spring  354  during compression. 
         [0068]    As shown in  FIG. 10B , as the bolt  310  moves into the slide bore  314 , the rotational portion  318   c  rotates the bolt  310  by applying torque to the bolt guide member  310   b . The bolt guide member  310   b  slides along the rotational portion  318   c  as the slide-carrier  302  moves forward. The rotation of the bolt head  310   a  locks the bolt  310  relative to the barrel extension  406 , providing a sealed chamber in which to fire a bullet. The integrated slide-carrier  302  then continues moving toward the barrel extension  406  while the bolt remains stationary and locked, as shown in  FIG. 10C . The integrated slide-carrier  302  continues moving toward the barrel extension because the bolt  310  should be fully rotated and locked relative to the barrel extension  406  before the firing pin  308  (visible in  FIG. 11A-C ) is positioned adjacent the bullet. 
         [0069]      FIGS. 11A-C  depict the same process in a cross-section view to show the compression of the firing pin spring  354  and the movement of the integrated slide-carrier  302  and firing pin  308  relative to the bolt  310 . The catch  318   b  retains the bolt  310  and urges the bolt  310  rearward during rearward motion of the integrated slide-carrier  302  and assists in aligning the bolt head  310   a  with the barrel extension  406  (barrel extension  406  not depicted in  FIGS. 11A-C ). Upon forward motion of the operating group  300  toward the barrel extension  406 , the bolt  310  contacts the barrel extension  406  first and the integrated slide-carrier  302  continues moving forward, compressing a firing pin spring  354  and pushing the bolt  310  into the slide bore  314 . 
         [0070]    As shown in  FIG. 11B , as the bolt  310  moves into the slide bore  314 , the rotational portion  318   c  rotates the bolt  310  by applying torque to the bolt guide member  310   b . The bolt guide member  310   b  slides along the rotational portion  318   c  as the slide-carrier  302  moves forward. The rotation of the bolt head  310   a  locks the bolt  310  relative to the barrel extension  406 , providing a sealed chamber in which to fire a bullet. The integrated slide-carrier  302  continues moving toward the barrel extension  406  while the bolt remains stationary and locked, as shown in  FIG. 11C . The integrated slide-carrier  302  continues moving toward the barrel extension because the bolt  310  should be fully rotated and locked relative to the barrel extension  406  before the firing pin  308  is positioned adjacent the bullet. 
         [0071]    As can be seen in  FIG. 11 , the firing pin spring  354  applies a force to the bolt  310  and the firing pin  308  that urges the two apart. Because the bolt  310  is locked relative to the barrel extension  406 , the firing pin spring  354  urges the firing pin  308  away from the bolt  310  and rearward in the slide bore  314 . However, the rearward travel of the firing pin  308  is limited by a bore cross-pin  330  and/or by the firing block  306 , itself. The firing pin  108  is urged away from the bolt head  310   a  and, therefore, away from the bullet B held in the chamber. The firing pin  308  has a degree of travel around the bore cross-pin  330 , however, which may be less than about 2 mm, less than about 1.5 mm, or less than about 1 mm. The force applied by the firing pin spring  354  to urge the firing pin  308  away from the bolt  310  and rearward in the bore  314  may also urge the firing block  306  rearward. As the firing block  306  moves rearward within the rear channel  334 , at least part of the firing block  306  protrudes from the integrated slide-carrier  302  or is otherwise configured to receive an impulse from a trigger package  508 . The protruding portion of the firing block  306  includes the hammer contact surface  344 . 
         [0072]    As shown in  FIGS. 12A-B , once in battery, the operating group  300  is ready to transmit an impulse from the trigger package  508  to a bullet B. The hammer contact surface  344  protrudes from the rear channel  334  and the firing pin contact surface  342  may be in contact with or adjacent to the firing pin  308 . The firing pin  308  rests on the bore cross-pin  330  and is held there by a force applied between the bolt  310  and the firing pin  308  by the firing pin spring  354 . As depicted in  FIG. 12A , when resting on the bore cross-pin  330  due to a rearward force applied by the firing pin spring  354 , a tapered end of the firing pin  308   a  may be substantially flush with a surface of the bolt head  310   a  or may be recessed therefrom. The tapered end of the firing pin  308   a  may, therefore, by adjacent or proximate a bullet B. 
         [0073]      FIG. 12B  shows a movement of the firing pin  308  in response to an impulse provided by a trigger package  508 . The impulse may be provided by a hammer  510  moving in a substantially arcuate fashion, as shown in  FIG. 12B , a striker moving in a substantially linear fashion, or any other mechanical impulse source configured to trigger an impact or impulse explosive such as the primer in a bullet B. In an embodiment, the impulse is delivered by a curved hammer  510 , such as that depicted in  FIGS. 12A-B . In a further embodiment, the impulse may be delivered by a HECKLER AND KOCH hammer operated trigger package. In a yet further embodiment, the impulse may be delivered by a HECKLER AND KOCH hammer operated trigger package that is substantially unmodified. In a still yet further embodiment, the impulse may be delivered by a HECKLER AND KOCH hammer operated trigger package that is modified only to remove the ejector from the trigger package. In an embodiment, the firearm  100  is a HECKLER AND KOCH host. 
         [0074]    The impulse is received by a hammer contact surface  344  of the firing block  306  and transmitted by the firing block  306  to a firing pin  308  through a firing pin contact surface  342  of the firing block  306 . Upon receiving the impulse, the firing block  306  slides forward on the guide pins  340 , moving substantially coaxially to the application of the impulse. The impulse source from the trigger package  508  may remain in contact with the firing block  306  while the firing block  306  contacts the firing pin  308 , or the impulse source may strike the firing block and, after imparting energy to the firing block  306 , retract from the firing block  306 . In an embodiment, the impulse source from the trigger package  508  applies a force to the firing block  306  and continues applying a force to the firing block  306  even after the firing block  306  travels forward and pushes the firing pin  308  forward. 
         [0075]      FIG. 13A  shows the operating group  300  and the trigger package  508  in the short time immediately following the combustion of the propellant in the bullet B. After the trigger package  508  has provided an impulse to the operating group  300 , and, particularly, the hammer contact surface  344  of the firing block  306 , to fire a bullet B, the expanding gas will impinge upon the gas piston  604  (not depicted in  FIGS. 13A-C ) and apply a rearward force on the operating rod  606 , which is coupled to the operating rod connection  312  of the integrated slide-carrier  302 . The force drives the operating group  300  rearward on the receiver rails  206   a ,  206   b  (not depicted) and the resulting rearward motion of the integrated slide-carrier applies a rearward force to the impulse source of the trigger package  508 . For example, the impulse source may be a hammer  510 , as depicted in  FIG. 13A , but may also be a striker or other linear impulse source. When the integrated slide-carrier  302  moves rearward relative to the trigger package  508 , the hammer  510  will be also urged rearward. The hammer  510  moves within a substantially arcuate path, and therefore, moving the hammer  510  rearward will cause the hammer  510  to also move toward the trigger package  508  and out of the rearward path of the operating group  300 . 
         [0076]    As shown in  FIG. 13B , a bevel  326  disposed on a portion of the integrated slide-carrier  302  nearest the hammer  510  aids in directing the hammer  510  out of the path of the integrated slide-carrier  302  and toward the trigger package  508  and housing  502 . In an alternative embodiment, the bevel  326  may alternatively be a rounded corner of the integrated slide-carrier  302  such that the rounded corner also provides a gradual and lower friction application of force to the hammer  510  or other impulse source in order to reset the hammer  510  or other impulse source, as depicted in  FIG. 13C , with an increased efficiency versus an integrated slide-carrier  302  with a squared corner. The lower support  320  holds the hammer  510  or other impulse source in its reset position for substantially the entire motion of the operating group  300  during the cycling of the firearm  100  in order to give the trigger package  508  as much time as is available to safely reset the trigger and prevent additional automatic firing, be it a single round or a “runaway” firearm, or to prevent the hammer  510  merely following the operating group  300  forward and failing to impart a sufficient impulse to detonate a primer. When in fully automatic firing mode, the sear catch arm  324  engages a sear on an appropriate fully automatic trigger package  508  and allows for a delayed release of the hammer  510  or other impulse source. The delayed release of the hammer  510  or other impulse source ensures the impulse is sufficient to detonate a primer. 
         [0077]    Referring now to  FIGS. 14A-C , the fire mode selector switch  512  is mounted on the housing  502  and trigger package  508 , and selects the fire mode for the trigger package  508 . While a three-position fire mode selector switch  512  is depicted in  FIGS. 14A-C , a number of trigger packages  508  are commercially available, including variants that may include more than three positions. As shown in  FIG. 13A , a counterclockwise-most position of the three-position fire mode selector switch  512  is a “disassemble” position. When the fire mode selector switch  512  is in the counterclockwise-most position, it may be removed from the housing  502  and from the trigger package  508 . The fire mode selector switch  512  is the only connection that retains the trigger package  508  in the housing  502 . Therefore, when the fire mode selector switch  512  is removed from the housing  502  and trigger package  508 , there are no further connections holding the trigger package  508  in place, and the trigger package  508  is free to move within the housing  502  and within the receiver body  202 . 
         [0078]    As can be seen in  FIG. 14B , to prevent accidental removal of the fire mode selector switch  512  when the firearm  100  is assembled, a selector stop  210  is disposed on the receiver body  202  such that the “disassemble” position may not be achieved when the control assembly  500  is attached to the receiver  200 . The fire mode selector switch  512  is depicted in a second position in  FIG. 14B . The second position is substantially rotationally adjacent the selector stop  210 . In an embodiment, the second position may be a “safe” mode, in which the trigger package  508  is inhibited from releasing the hammer  510  or other impulse source and the firearm  100  is therefore unable to fire. In another embodiment, the second position may be a firing mode, and the firing mode may include a semi-automatic, burst-fire, or fully-automatic firing mode. 
         [0079]      FIG. 14C  depicts a third position of the fire mode selector switch  512 , which is rotationally further from the selector stop  210  than the second position. In an embodiment, the third position may be a “safe” mode, in which the trigger package  508  is inhibited from releasing the hammer  510  or other impulse source and the firearm  100  is therefore unable to fire. In another embodiment, the third position may be a firing mode, and the firing mode may include a semi-automatic, burst-fire, or fully-automatic firing mode. 
         [0080]      FIG. 15  depicts an exploded view of the removable trigger package  508  from the grip housing  502 . Fire mode selector switch shaft  514  extends the width of the housing  502 . When the trigger package  508  is disposed within the housing  502 , housing port  516  aligns with trigger package port  518 , and fire mode selector switch shaft  514  may be inserted through the width of the housing  502  and the trigger package  508  to secure the trigger package  508  within the housing  502 . 
         [0081]    When the fire mode selector switch  512  rotates to the “disassemble” position depicted in  FIG. 14A , the fire mode selector switch  512  may be removed. There is no other connection between the trigger package  508  and the grip housing  502  securing the trigger package  508  in the grip housing  502 . Therefore, upon removal of the fire mode selector switch  512  (by lateral movement of the fire mode selector switch  512 ) from the grip housing  502  and the trigger package  508 , the trigger package  508  is no longer secured to any part of firearm  100 . 
       6.8 mm SPC Conversion Kit 
       [0082]    While a closed-bolt variant has been described herein, a 6.8 mm SPC Conversion Kit as described in the present disclosure may convert the more common open-bolt variants of the M249 platform. An open-bolt M249 platform firearm is depicted in  FIG. 22 . An M249 platform firearm may be adapted to fire 6.8 mm SPC ammunition by adapting a barrel assembly  400  and a bolt  310 , as shown in  FIG. 1  (a closed-bolt variant of the M249 platform) and  FIG. 22  (an open-bolt variant of the M249 platform). Converting the barrel assembly  400  includes converting at least a diameter of the barrel bore  404 , a diameter of the gas port  410 , and the diameter of the chamber  414 . Converting the bolt  310  includes converting at least the diameter of the bolt face  356  and the size of the extractor  358 . While reference may be made to components depicted in one or more Figures showing a closed-bolt variant, any components described in relation to a 6.8 mm SPC Conversion Kit are common to all M249 platform firearms and should be understood to be non-limiting as to the type of firearm within the M249 platform. The presently described 6.8 mm SPC Conversion Kit is applicable at least to the FN MINIMI, DOD-designation M249, MK46, MK48, and the MGA SAW. 
         [0083]    As shown in  FIG. 16 , a 6.8 mm SPC Conversion Kit  800  includes a barrel  802  having a gas port  810  therein, a barrel extension  806 , and a bolt  910 . A barrel bore  804  extends through a length of the barrel  802 . The barrel bore  804  is in communication with a chamber  814 . The barrel bore  804  is also in fluid communication with a gas port  810 . The gas port  810  is covered by the gas block  808  and the gas block  808  redirects the gas from the barrel bore  804  laterally away from the barrel bore  804  and toward the gas block outlet  812 . 
         [0084]    Referring now to  FIG. 17 , the bolt  910  is adapted to receive 6.8 mm SPC ammunition. In particular, the bolt face  956 , a recessed portion of the front of the bolt head  910   a , is adapted to receive and secure an SPC ammunition casing. In order to receive and secure the SPC casing, which has a base diameter of about 0.422 inches, the bolt face  956  has a diameter D BF  that is larger than the casing diameter and sufficient to allow the casing to be rotated during ejection of the spent brass after firing. In an embodiment, the D BF  of the bolt face  956  is greater than about 0.422 inches. In another embodiment, the D BF  of the bolt face  956  is about 0.422 inches to about 0.460 inches. In yet another embodiment, the D BF  of the bolt face  956  is about 0.428 inches. In yet a further embodiment, the D BF  of the bolt face  956  is greater than about 0.422 inches and is not more than about 0.432 inches, e.g., about 0.423 inches to about 0.432 inches. In yet another embodiment, the D BF  of the bolt face  956  is within a range of about 0.422 inches to about 0.457 inches. In yet another embodiment, the D BF  of the bolt face  956  is within a range of about 0.422 inches to about 0.452 inches. In yet another embodiment, the D BF  of the bolt face  956  is within a range of about 0.422 inches to about 0.450 inches. In a yet further embodiment, the D BF  of the bolt face  956  is within a range of about 0.422 inches to about 0.432 inches. 
         [0085]    As shown in  FIGS. 17-18 , the casing of bullet B is retained within the recessed bolt face  956  by the extractor  958 . The extractor  958  is urged radially inward toward the center of the bolt face  956 . The extractor  958  may be urged radially by a spring positioned radially against the extractor  958 . The extractor  958  may be urged radially by an arm positioned longitudinally, wherein the flexion of the arm provides a radial force inward to the extractor  958 . The firing pin  908  may extend through a centerpoint of the bolt face  956  for proper detonation of the centerfire ammunition. After firing, the bullet B is retained by the extractor  958  having a rearward-biased hook disposed toward and configured to engage with the casing of bullet B. In the depicted embodiment, the hook has a height configured to mate with an indention in the rear of the casing of an SPC bullet. In another embodiment, the hook may have a height of about 0.358 inches. After firing, as the high pressure gas impinges upon a gas piston and drives an operating group rearward, the extractor  958  draws the casing of bullet B rearward and out of the chamber  814  before the casing is ejected from the firearm. 
         [0086]      FIG. 19  depicts the barrel  802  and barrel extension  806  forming the chamber  814  that is closed on the rearward end when the bolt head  910   a  locks relative to the barrel extension  806  as described early and in  FIGS. 10A-11C . The diameter of the bore D B  is sufficient to allow passage of the 6.8 mm SPC bullet therethrough while maintaining accuracy. In an embodiment, the diameter of the bore D B  is greater than about 0.270 inches. In another embodiment, the diameter of the bore D B  is greater than about 0.277 inches. In yet another embodiment, the diameter of the bore D B  is about 0.270 inches. 
         [0087]    The chamber  814  may have a length substantially equivalent to the length of the casing of SPC ammunition. The length of the chamber from the breach face to the furthest point to which a casing may extend beyond the breach is between about 1.6044 inches and about 1.7609 inches. In another embodiment, the length of the chamber may be about 1.7109 inches. In particular, the distance from the shoulder of the bullet to the breach face may be between about 1.280 inches and about 1.300 inches. In another embodiment, the distance from the shoulder of the bullet to the breach face may be about 1.296 inches. The chamber also has a diameter D C  that is configured to retain an SPC round therein during firing, but also facilitate reliable removal by the extractor  358  after firing. In an embodiment, the chamber has a diameter larger than that of an SPC round. In another embodiment, the chamber has a diameter greater than about 0.4207 inches. In a further embodiment, the chamber has a diameter about 0.422 inches. 
         [0088]    As shown in  FIG. 20 , the diameter of the gas port  810  partially determines the gas pressure at the gas block outlet  812 . Therefore, the diameter of the gas port  810  is tuned to provide the proper amount of gas pressure in order to properly cycle the operating group of the firearm. The gas pressure in the barrel bore will typically increase with the size and energy of the ammunition used. The energy delivered by the expanding gas may vary depending on length of the barrel. In an embodiment, to properly cycle the operating group of an open-bolt M249 platform firearm, the gas port  810  may be about 0.062 inches to about 0.150 inches in diameter. In another embodiment, the gas port  810  may be about 0.082 inches to about 0.140 inches in diameter. In yet another embodiment, the gas port  810  may be about 0.085 inches to 0.101 inches in diameter. In yet another embodiment, the gas port  810  may be about 0.090 inches to about 0.100 inches. In yet another embodiment, the gas port  810  may be about 0.093 inches in diameter. 
         [0089]      FIG. 21  depicts a feed tray  702  configured to receive and transfer belted 6.8 mm SPC ammunition from a storage location (such as a drum, bag, or box) to the chamber  814  provided by the 6.8 mm SPC conversion kit  800 . The feed tray  702  may include a flared portion  704  that guides the belted 6.8 mm SPC ammunition into a channel  708  having a longitudinal length  706 . In the depicted embodiment, the longitudinal length  706  is about 2.3 inches. 
         [0090]      FIG. 22  depicts an open-bolt machine gun  102  that may be converted to fire 6.8 mm SPC ammunition using the 6.8 mm SPC conversion kit  800  of  FIG. 16 . As described herein, the 6.8 mm SPC conversion kit  800  may convert either an open-bolt machine gun  102  or a closed-bolt machine gun  100  of the M249 platform. The open-bolt machine gun  102  has a barrel assembly  400  similar to that described in relation to a closed-bolt variant. The barrel assembly  400  depicted in  FIG. 22  is exchanged for the barrel  802  and barrel extension  806  (described in relation to  FIG. 16  and  FIGS. 19 and 20 ) and the bolt  310  is exchanged for the bolt  910  having bolt face  956  and extractor  958  (described in relation to  FIGS. 17 and 18 ). 
         [0091]    Thus, a kit for converting an open-bolt or closed-bolt M249-platform firearm to allow the use of 6.8 mm SPC ammunition therein includes a barrel having a bore, chamber, and gas port adapted for the 6.8 mm SPC ammunition; a bolt having a bolt face and extractor adapted for the 6.8 mm SPC ammunition; and a barrel extension adapted for the 6.8 mm SPC ammunition as discussed in connection with  FIGS. 16-20 . 
         [0092]    The articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements in the preceding descriptions. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Numbers, percentages, ratios, or other values stated herein are intended to include that value, and also other values that are “about” or “approximately” the stated value, as would be appreciated by one of ordinary skill in the art encompassed by embodiments of the present disclosure. A stated value should therefore be interpreted broadly enough to encompass values that are at least close enough to the stated value to perform a desired function or achieve a desired result. The stated values include at least the variation to be expected in a suitable manufacturing or production process, and may include values that are within 5%, within 1%, within 0.1%, or within 0.01% of a stated value. 
         [0093]    A person having ordinary skill in the art should realize in view of the present disclosure that equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations may be made to embodiments disclosed herein without departing from the spirit and scope of the present disclosure. Equivalent constructions, including functional “means-plus-function” clauses are intended to cover the structures described herein as performing the recited function, including both structural equivalents that operate in the same manner, and equivalent structures that provide the same function. It is the express intention of the applicant not to invoke means-plus-function or other functional claiming for any claim except for those in which the words ‘means for’ appear together with an associated function. Each addition, deletion, and modification to the embodiments that falls within the meaning and scope of the claims is to be embraced by the claims. 
         [0094]    The terms “approximately,” “about,” and “substantially” as used herein represent an amount close to the stated amount that still performs a desired function or achieves a desired result. For example, the terms “approximately,” “about,” and “substantially” may refer to an amount that is within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of a stated amount. Further, it should be understood that any directions or reference frames in the preceding description are merely relative directions or movements. For example, any references to “up” and “down” or “proximal” or “distal” are merely descriptive of the relative position or movement of the related elements. 
         [0095]    The present disclosure may be embodied in other specific forms without departing from its spirit or characteristics. The described embodiments are to be considered as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. Changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.