Patent Publication Number: US-11022389-B2

Title: Gas operating system for an automatic firearm

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/620,290 titled GAS OPERATING SYSTEM FOR AN AUTOMATIC FIREARM and filed on Jan. 22, 2018, the contents of which are incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates to firearms, and more particularly to gas-operated firearms with automatic-firing capabilities. 
     BACKGROUND 
     During the firing of a firearm, combustion gases move the bullet through the barrel until it exits the bore at the muzzle-end of the barrel. Gas-operated firearms include a gas port located along the barrel of the firearm to receive combustion gases produced during the firing cycle. Pressurized gases enter the gas port to automatically reload the firearm. Movement of system components causes a spent cartridge to be ejected from the chamber of the firearm, and a new cartridge to be subsequently loaded therein. With the new cartridge loaded, the firearm is readied for the next firing cycle. 
     SUMMARY 
     The present disclosure provides a gas operating system for a firearm. In accordance with one embodiment, a gas operating system includes a barrel having a bore that includes a rifled portion of a first diameter. A gas block is attached to the distal end portion of the barrel. The gas operating system defines a gas expansion chamber located distally of the rifled bore portion, where the gas expansion chamber is in fluid communication with the bore and the gas block. The present disclosure also provides a firearm including the gas operating system, in accordance with some embodiments. For example, the firearm is chambered for 5.56×45 mm ammunition and has a barrel length from five to nine inches. The gas operating system can include a piston configured to cycle the action of the firearm in response to gas pressure generated during the firing cycle. Numerous variations and configurations will be apparent in light of the present disclosure. 
     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 inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side view of a firearm including a gas operating system configured in accordance with an embodiment of the present disclosure. 
         FIG. 1B  is a side view of a firearm shown in  FIG. 1A  with a handguard removed to expose a gas operating system, in accordance with an embodiment of the present disclosure. 
         FIG. 2  is a perspective view of an operating system, in accordance with an embodiment of the present disclosure. 
         FIG. 3A  is a perspective view of a barrel of the firearm, in accordance with an embodiment of the present disclosure. 
         FIG. 3B  is a cross-sectional view of the barrel shown in  FIG. 3A , in accordance with an embodiment of the present disclosure. 
         FIG. 4A  is a perspective view of a gas block of a gas operating system, in accordance with an embodiment of the present disclosure. 
         FIG. 4B  is a cross-sectional view of the gas block shown in  FIG. 4A , in accordance with an embodiment of the present disclosure. 
         FIG. 5A  is a perspective view of an insert of a gas operating system, in accordance with an embodiment of the present disclosure. 
         FIG. 5B  is a cross-sectional view of the insert shown in  FIG. 5A , in accordance with an embodiment of the present disclosure. 
         FIG. 6  is a cross-sectional view of a gas operating system, in accordance with an embodiment of the present disclosure. 
         FIG. 7  is an enlarged view of a gas expansion chamber of the gas operating system shown in  FIG. 6 , in accordance with an embodiment of the present disclosure. 
         FIG. 8A  is a perspective view of a gas operating system for a firearm, in which the insert is within the gas block, in accordance with an embodiment of the present disclosure. 
         FIG. 8B  is a cross-sectional view of the gas operating system shown in  FIG. 8A , in accordance with an embodiment of the present disclosure. 
         FIG. 9A  is a perspective view of a gas operating system for a firearm, in which the air chamber is within the barrel, in accordance with an embodiment of the present disclosure. 
         FIG. 9B  is a cross-sectional view of the gas operating system shown in  FIG. 9A , in accordance with an embodiment of the present disclosure. 
         FIG. 10A  is a perspective view of a gas operating system for a firearm, in which the insert is external to the barrel, in accordance with another embodiment of the present disclosure. 
         FIG. 10B  is a cross-sectional view of the gas operating system shown in  FIG. 10A , in accordance with an embodiment of the present disclosure. 
         FIG. 11A  is a perspective view of a gas operating system without a removable insert for a firearm, in accordance with another embodiment of the present disclosure. 
         FIG. 11B  is a cross-sectional view of the gas operating system shown in  FIG. 11A , in accordance with an embodiment of the present disclosure. 
         FIG. 12A  is a photograph of an opening of a gas port within a barrel of a firearm having a conventional gas operating system. The photograph was recorded prior to firing projectile rounds with the firearm. 
         FIG. 12B  is a photograph that illustrates erosion of the opening of the gas port shown in  FIG. 12A  after firing 1,200 rounds of ammunition with the firearm. 
         FIG. 12C  is a photograph that illustrates erosion of the opening of the gas port shown in  FIG. 12A  after firing 2,400 rounds of ammunition with the firearm. 
         FIG. 12D  is a photograph that illustrates erosion of the opening of the gas port shown in  FIG. 12A  after firing 3,600 rounds of ammunition with the firearm. 
         FIG. 12E  is a photograph that illustrates erosion of the opening of the gas port shown in  FIG. 12A  after firing 4,800 rounds of ammunition with the firearm. 
         FIG. 12F  is a photograph that illustrates erosion of the opening of the gas port shown in  FIG. 12A  after firing 6,000 rounds of ammunition with the firearm. 
         FIG. 13A  is a photograph showing an angled view of an opening of a gas port of a gas operating system after firing 5,000 rounds of ammunition, in accordance with an embodiment of the present disclosure. 
         FIG. 13B  is another photograph showing the opening of the gas port shown in  FIG. 13A  as viewed looking into the gas port, in accordance with an embodiment of the present disclosure. 
     
    
    
     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 
     Techniques and architectures are disclosed for a gas operating system for a firearm, such as a short-barreled automatic rifle. The gas operating system is configured to utilize combustion gases generated by a cartridge during firing to automatically ready the firearm for its next firing cycle. The system includes a barrel having a bore through which a projectile can pass. The bore is rifled along its length to its opening at a distal end, in accordance with some embodiments. A gas block is attached to the distal end of the barrel. The gas block includes a piston and a gas port that communicates with the bore. In response to receiving pressurized gases from the gas port, the piston moves to cycle the action. Distally adjacent to the rifled portion of the bore, and axially aligned with the bore, is a chamber, for example gas expansion chamber. Combustion gases from the barrel enter the gas expansion chamber where they cool and expand to some extent after leaving the distal end of the barrel. The expansion of the combustion gases within the gas expansion chamber decreases the pressure, temperature, and/or velocity of the gases, and thereby reduces gas port erosion that adversely affects firearm durability, accuracy and/or performance of the firearm. Thus, the gas operating system in accordance with some embodiments of the present disclosure enables the firearm to achieve or exceed its designed service life by reducing or otherwise eliminating gas port erosion that necessitates repair of the firearm. 
     General Overview 
     As discussed above, gas-operated firearms can include a gas port within the barrel. The gas port, however, can erode or otherwise deform over time after repeated firing cycles. Gas port erosion is a particular concern for short-barreled firearms (e.g., rifles having barrels of less than nine inches in length) that fire rifle-caliber ammunition (e.g., 5.56×45 mm ammunition). Erosion of the gas port significantly reduces the life of the barrel, and thereby necessitates its repair or replacement at an earlier stage. In more detail, a shorter barrel means that the combustion gases enter the gas port at a higher pressure, velocity and/or temperature because the gas port is located closer to the cartridge chamber than in long-barreled rifles. In addition, the rifle-caliber cartridges produce combustion gases at greater pressures, temperatures, and/or velocities than smaller cartridges (e.g., pistol cartridges) because the rifle cartridge include a larger propellant charge. Together, the shorter barrel lengths and firing rifle cartridges cause high pressure, high velocity, and/or high temperature combustion gases to enter the gas port, and thereby rapidly wear down surfaces of the gas port. In some instances, for example, the erosion of the gas port can damage the rifling within the barrel, for example by creating a void in the rifling. The void in the rifling causes an interruption within the rifling of the barrel, in which the projectile is unsupported. As a result, contact between the projectile and damaged rifled surfaces damages the jacket of the projectile as it moves through the barrel. In turn, the damaged jacket causes the projectile to fly inaccurately through the air once the projectile exits the barrel. In other instances, erosion of the gas port can increase its effective diameter, and thereby allow more combustion gases through the port. The additional combustion gases increase the pressure acting on the piston of the gas operating, causing the gas operated system to cycle more quickly and with greater force than designed. As a result, gas operating system components wear more quickly and need replacement because the faster moving components apply greater force against each other. 
     Thus, and in accordance with an embodiment of the present disclosure, techniques and architectures are disclosed for a gas operating system, for example a direct impingement or gas piston system, for a firearm, such as a short-barreled automatic rifle. The gas operating system in accordance with some embodiments of the present disclosure is configured to utilize combustion gases generated by a cartridge during firing to automatically ready the firearm for its next firing cycle. In more detail, the system can include a barrel (e.g., a barrel with a length of 5.5 inches) having a bore including a rifled portion that provides a pathway for a projectile (e.g., a 5.56 mm rifle projectile). Attached to the barrel is a gas block that provides fluid communication between the barrel and the piston of the gas operating system. The gas block, in some examples, is adjacent to a muzzle-end of the barrel. The gas block can be concentric about a portion of the barrel, such that the overall length of the firearm is not increased by installation of the gas block thereon. 
     In one embodiment, the gas block includes a lower cylinder and an upper cylinder. The lower cylinder is configured to receive a portion of the barrel. For example, the lower cylinder is configured to receive the barrel, such that the barrel extends through the lower cylinder and extends beyond the distal end of the gas block&#39;s lower cylinder. In some other examples, the lower cylinder is further configured to receive an insert, for example a flash hider or suppressor. The upper cylinder of the gas block, on the other hand, is configured to receive a piston and a valve, such that the piston can move within the gas block to initiate reloading of the firearm. Disposed between the upper and lower cylinders is a gas port, such as a gas block port. The gas block port is configured to supply combustion gases generated by the cartridge to the valve to move the piston to cycle the gas operating system. 
     Adjacent to the rifled portion of the bore is a chamber, for example a gas expansion chamber. The gas expansion chamber can be aligned with the bore axis. The gas expansion chamber receives combustion gases from the bore and supplies the gases to the gas port to cycle the gas operating system. The gas expansion chamber is further configured with sufficient volume to allow the combustion gases to expand and flow within the chamber before entering the gas port. The expansion of the combustion gases within the chamber decreases the pressure, temperature, and/or velocity of the gases, and thereby reduces gas port erosion that adversely affects firearm accuracy and/or performance. The gas expansion chamber, in some examples, can be integrated within the barrel of the firearm such that a volume of the chamber is defined by the barrel. To this end, in some examples, the gas expansion chamber includes an average diameter that is at least two, three or four times the bore diameter of the barrel. In some cases, the maximum diameter of the chamber can be two, three or four times the diameter of the bore. In some cases, the axial length of the gas expansion chamber along the bore axis can be less than the length of the projectile that passes through the barrel. 
     In yet other examples, the gas expansion chamber can have a volume defined by a combination of the barrel (e.g., the muzzle-end of the barrel), the gas block, and/or an insert. In one example, the insert is axially aligned with the barrel and the gas block to form a pathway in which the projectile can travel to exit the firearm. Moreover, the insert can be removably attached to either the barrel or the gas block, depending on a given application. In addition, the insert can be further configured to enable flow of gases within the gas expansion chamber to generate regions of reduced pressure, temperature, and/or gas velocity. For instance, the insert may include one or more control surfaces that re-direct the flow of gases as they expand within the gas expansion chamber. In some examples, this may include guiding the gas flow within the gas expansion chamber so as to limit or otherwise prevent high-pressure gases that enter the gas expansion chamber from enveloping or otherwise surrounding the projectile as the projectile moves through the gas expansion chamber. This can be significant, because high-pressure gases that impinge upon the projectile can cause the projectile to move off its intended trajectory, thereby reducing accuracy of the firearm. In addition, the insert can be configured or otherwise positioned relative to the barrel such that part of the projectile enters the insert before the entire projectile completely exits the barrel. In other words, the projectile can bridge the gap between the barrel and the insert so as to prevent the expansion of high-pressure combustion gases within the gas expansion chamber from surrounding or otherwise engulfing the projectile. Such feature can be used to maintain firearm accuracy while the projectile moves through the firearm. In addition, the insert may be further configured to receive additional components (e.g., a flash hider or suppressor) to enhance firearm performance. Numerous other gas operating system configurations will be apparent in light of the present disclosure. 
     Example Firearm Application 
       FIG. 1A  is a side view of a firearm  100  including a gas operating system  120  configured in accordance with an embodiment of the present disclosure.  FIG. 1B  is a side view of the firearm shown in  FIG. 1A  with the handguard  112  removed. In one example, the firearm  100  is an automatic rifle, such as a short-barreled automatic rifle. The firearm  100 , in some examples, is configured to fire rifle cartridges, such as 5.56 NATO rifle ammunition. As can be seen, in this one example, the firearm  100  includes a lower receiver  105  and an upper receiver  110 . The lower receiver  105  assembles to the upper receiver  110  and may include other rifle components, such as components of the fire control group. Other components of the firearm  100 , such as a handguard  112 , a barrel  115  and a gas operating system  120 , are assembled on the upper receiver  110 . During the firing cycle, the projectile is propelled through the barrel  115  by combustion gases generated therein. As the projectile passes through the barrel  115 , some of the combustion gases are diverted from the barrel  115  to operate the gas operating system  120 , which cycles the firearm  100  action for the next firing cycle, as will be described further below. 
     Example Gas Operating System Configuration 
       FIG. 2  is a perspective view of a gas operating system  120 , in accordance with an embodiment of the present disclosure. In this example, the gas operating system  120  is disposed on a distal end of the barrel  115  and secured with pins  225 . As can be seen, the system  120  includes a gas block  205 , a valve  210 , a piston  215 , and an insert  220 , each of which is described further herein. In general, the valve  210  within the gas block  205  receives some combustion gases as the projectile exits the rifled portion of the barrel  115  and directs the combustion gases to the piston  215 . In turn, the combustion gases impinge upon the piston  215  causing it to move. As the piston  215  moves in response to pressurized gases entering the valve (e.g., rearward), it contacts a bolt carrier to cycle the firearm  100 , eject the spent cartridge, and load another cartridge into the barrel  115 . 
       FIG. 3A  is a perspective view of a barrel  115  of the firearm  100 , in accordance with an embodiment of the present disclosure.  FIG. 3B  is a cross-sectional view of the barrel  115  shown in  FIG. 3A . In this example, the barrel  115  is a short-rifle barrel having a length of 5 inches. In other examples, the barrel  115  can have a length that ranges from less than 5 inches to greater than or equal to 9 inches, depending on a given application. For example, the barrel can have a length of 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, or 9 inches. 
     As can be seen, the barrel  115 , in some examples, includes a barrel body  305 , a bore  310 , a gas block attachment surface  315 , and a groove  320 . The barrel body  305  has a tubular shape made from high-strength materials, such as alloy-steel. Within the body  305  is a bore  310  through which the projectile travels. As can be seen, the bore  310  includes a chamber  325  configured to receive the cartridge at the proximal end portion of the barrel  115 . Adjacent to the chamber  325  is a rifled portion  330  through which the projectile moves before it exits the barrel  115  at bore opening  335 . On the exterior of the barrel body  305  is a gas block attachment surface  315  configured to receive a portion of the gas block  205  (e.g., a cylinder  415 , as described further below). In some examples, the surface  315  can be a smooth cylindrical surface having a reduced diameter as compared with other portions of the barrel  115 . The surface  315 , in some examples, further includes the groove  320  configured to receive a seal (e.g., an O-ring) to prevent combustion gases from exiting the gas operating system  120  through the joint between the barrel  115  and the gas block  205 . 
       FIG. 4A  is a perspective view of a gas block  205  of the gas operating system  120 , in accordance with an embodiment of the present disclosure.  FIG. 4B  is a cross-sectional view of the gas block  205  shown in  FIG. 4A . In general, the gas block  205  is configured to receive combustion gases from the barrel  115  and guide or otherwise direct some of those gases to operate a piston disposed therein (e.g., piston  215 ). The gas block  205  provides a fluid flow path for combustion gases to cycle the firearm  100  and automatically ready the firearm  100  for another firing cycle. In this illustrated example, the gas block  205  includes a gas block body  405  defining an upper cylinder  410  and a lower cylinder  415  therethrough, and a gas block port  420 . In a general sense, the body  405  is a housing or structure that interfaces with the firearm  100  (e.g., attaches to the barrel  115 ) and supports components of the system  120  (e.g., the piston  215 ). The body  405  can be made from high-strength materials, such as 17-4 PH stainless steel, to withstand the forces applied to the body  405  by the combustion gases present therein during the firing cycle. As can be seen, the body  405  includes the upper cylinder  410  in which to receive components of the gas operating system  120 , such as the valve  210  and the piston  215 . In some examples, the upper cylinder  410  includes a piston stop surface  412  that limits movement of the piston  215  at one end of its travel. In some examples, upper cylinder  410  need not include the valve  210  and the piston  215 . Rather, the gas operating system  120  can be configured as a direct impingement system. In such instances, gases from the cylinder  410  (e.g., via a port in the cylinder and gas tube connected thereto) contact or otherwise impinge upon components of a bolt carrier mechanism to reload the firearm. 
     Below the upper cylinder  410  is the lower cylinder  415  configured to receive the distal end portion of the barrel  115 . In some examples, the lower cylinder  415  is further configured to receive the insert  220  at an opposing end. In some such configurations, the lower cylinder  415  includes a plurality of internal threads  418  (e.g., 13/16-UNEF threads) to secure the insert  220  to the gas block  205 . The lower cylinder  415  may also further include a tapered sealing surface  416 , to form a seal with a corresponding surface on the insert. The resulting seal prevents gases from exiting through the joint between the gas block  205  and the insert  220  when the insert  220  is installed thereon. As can be seen, the upper cylinder  410  can be aligned with the lower cylinder  415 , such that the cylinders  410  and  415  are directly above each other. In some other examples, one of the cylinders  410  or  415  can be offset from (e.g., at an angle of 45 degrees) or otherwise next to (e.g., side-by-side) the other, depending on a given application. 
     The lower cylinder  415 , in some examples, is further configured to position the insert  220  and the barrel  115  at a distance from one another so as to define a gas expansion chamber (e.g., gas expansion chamber  605  shown in  FIG. 6 ) that reduces pressure, temperature, and/or velocity of the combustion gases passing therethrough, as will be described further herein. In addition, the lower cylinder  415  may further include one or more raised features (e.g., bumps, steps, etc.) or recessed features (e.g., grooves, channels, dimples, recesses, etc.) or a combination thereof, that assist with generating one or more regions of lower pressure, temperature, and/or velocity of combustion gas within the gas expansion chamber. 
     Disposed between the upper cylinder  410  and the lower cylinder  415  is a gas block port  420 . The gas block port  420 , in general, is configured to receive combustion gases from the barrel  115  and re-direct those gases to the valve  210  disposed within the upper cylinder  410  to operate the gas operating system  120 . In this example, the gas block port  420  is a vertical internal bore that extends from the lower cylinder  415  to the upper cylinder  410 . In other examples, the gas block port  420  may be positioned at an angle relative to one of the cylinders  410  and  415 . The gas block port  420 , in some examples, includes a diameter of 0.125 inches. Numerous other gas block embodiments will be apparent in light of the present disclosure. 
       FIG. 5A  is a perspective view of an insert  220  of the gas operating system  120 , in accordance with an embodiment of the present disclosure.  FIG. 5B  is a cross-sectional view of the insert  220  shown in  FIG. 5A . Generally speaking, the insert  220  can improve gas flow to help stabilize the projectile as it exits the barrel  115 , in accordance with some embodiments. In addition, the insert  220  can include one or more external surfaces that re-direct the flow of the combustion gases within a chamber to reduce the pressure, temperature, and/or velocity of the gases therein. In some examples, the insert  220  can be further configured to modify or otherwise reduce the appearance of flash as the projectile and/or burning propellant exit the barrel  115 . In yet other examples, the insert is further configured to receive additional firearm components, such as a flash hider or suppressor. As can be seen, in this example, the insert  220  includes an insert body  505 , an internal bore  510 , and control surfaces  515 . The body  505  can be a unitary component, in which features (e.g., external threads  520  and flash suppressant features  535 ) and surfaces (e.g., control surfaces  515  and tapered sealing surfaces  525 ) are disposed thereon. As can be seen, the body  505  defines the internal bore  510  that receives the projectile from the barrel  115  and allows the projectile to exit the firearm  100 . In some examples, the bore  510  can have a diameter of 0.245 inches. In other examples, the bore  510  can include a diameter within the range of less than 0.240 to greater than or equal to 0.260 inches. Numerous variations and embodiments are acceptable, as will be appreciated. 
     The insert  220  further includes one or more control surfaces  515 . In general, the control surfaces  515  guide or otherwise re-direct flow of combustion gases exiting the barrel  115  and impinging on the control surfaces  515 . Control surfaces  515  function to provide one or more regions of lower pressure, temperature, and/or velocity of combustion gas within the gas expansion chamber defined between the barrel  115  and the insert  220 . The control surfaces  515  can be a single surface or a combination of multiple surfaces on the proximal end of the insert  220 . The combination of surfaces may include one or more radiuses to allow different surfaces to transition smoothly from one surface to another. In some examples, the control surfaces  515  include tapered surfaces that are positioned at an angle (α) relative to a bore axis  540 . In this one example, the control surfaces  515  include a straight tapered portion having a 30-degree angle relative to axis  540 . In other examples, tapered portions of the control surfaces  515  can be located at 10, 15, 20, 35, 45, 50, 60, 75, and 85-degree angle relative to the longitudinal axis  540 . In addition, the control surfaces  515  can include a uniform diameter or a varying diameter, depending on a given application. The control surfaces  515 , moreover, can include, in some examples, one or more raised features (e.g., bumps, steps, etc.) or recessed features (e.g., grooves, dimples, recesses, etc.) or a combination thereof, that promote favorable fluid dynamics. The control surfaces  515 , in some examples, can be configured and arranged such that upon installation of the insert  220  within the gas block  205 , the control surfaces  515  extend or otherwise project into a gas expansion chamber of the firearm (e.g., gas expansion chamber  605 ) to prevent high-pressure combustion gases within the gas expansion chamber from enveloping or otherwise surrounding the projectile as it moves through the gas expansion chamber. 
     The insert body  505 , in some examples, further includes external threads  520  (e.g., 13/16-UNEF threads) to attach the insert  220  in the lower cylinder  415  of the gas block  205 . Adjacent to external threads  520  is tapered sealing surface  525  configured to form a seal with the gas block  205 . The body  505 , in this one example, further includes a plurality of flash suppressant features  535  configured to reduce the appearance of muzzle flash (e.g., visible light) from the firearm  100 . As can be seen, the suppressant features  535  are disposed on the distal end of the insert  220 . In this one example, the insert  220  includes at least three flash suppressant features  535  that are evenly distributed about the bore axis  540 . Numerous other configurations of insert  220  will be apparent in light of the present disclosure. 
       FIG. 6  is a cross-sectional view of the gas operating system  120 , in accordance with an embodiment of the present disclosure.  FIG. 7  is an enlarged view of the gas expansion chamber  605  of the gas operating system  120  shown in  FIG. 6 . As can be seen, in this one example, the gas operating system  120  includes a gas expansion chamber  605 . Generally speaking, the gas expansion chamber  605  allows combustion gases that propel the projectile  15  through the barrel  115  to expand, and thereby generate one or more regions of reduced pressure, velocity, and/or gas temperature. Some of the gases within these regions are diverted or otherwise supplied to the valve  210  of the gas operating system  120  via the gas block port  420  within the gas block  205 . This may be particularly noteworthy, because reduced pressure, temperature, and/or velocity of combustion gases cause less erosion of the gas block port  420  over time, thereby increases the service life and/or accuracy of the firearm. 
     The gas expansion chamber  605  can include geometry that promotes movement of combustion gases therein. For instance, as shown in  FIG. 7 , the diameter “D” of the gas expansion chamber  605  can be longer than an axial length “C” of the gas expansion chamber  605 . In this example, the axial length “C” is the distance from the distal end of the barrel  115  to the location at which the insert  220  contacts the inside of lower cylinder of the gas block  205  to define one end of the gas expansion chamber  605 . In other embodiments, the axial length “C” of the gas expansion chamber  605  can be greater than the diameter “D”, depending on a given application. In some embodiments, the gas expansion chamber  605  can defined by a combination of surfaces, such as an inside diameter of the lower cylinder  415  of the gas block  205 , one or more control surfaces  515  of the insert  220 , and the distal face of the barrel  115 . In some cases, the gas expansion chamber  605  can have a cross-sectional shape (as viewed from the side as in  FIG. 7 ) of a cylinder, a polygon, a sphere, and a cone (or a combination thereof). In some such cases, the cross-sectional shape of the gas expansion chamber  605  may further include a frustum feature (e.g., surface perpendicular to the bore axis) at one or both ends of the chamber  605 . In addition, the gas expansion chamber  605  may include one or more tapered, angled, or otherwise curved surfaces that promote flow of combustion gases within the gas expansion chamber  605  to assist with projectile travel and/or to operate a gas operating system. Furthermore, the gas expansion chamber  605  may also include one or more radiuses at the interface between two or more components disposed therein to further promote the flow of combustion gases within the gas expansion chamber  605 . The cross-sectional shape can be consistent or can vary along the axial length of the chamber. In some cases, the shape and volume of the chamber is defined by the gas block  205  and insert  220 . 
     The size of the gas expansion chamber  605 , in some examples, can be based at least in part on the diameter of the bore  310  of the barrel  115  relative to the inner diameter of the lower cylinder  415  of the gas block  205 . For instance, as can be seen in  FIG. 7 , the gas expansion chamber  605  is located immediately adjacent the distal end of the barrel within a portion of the lower cylinder  415  of the gas block  205  having a diameter “D”. The diameter “D”, as shown, can be from 0.675 inches to 0.697 inches, depending on a given application. In other cases, the diameter “D” has a size in the range of 0.500 inches to 0.750 inches, depending on the application. The diameter “D”, in some cases, can be twice the size of a bore of diameter “B” of the barrel  115 . For instance, for a bore diameter “B” ranging in size between 0.225 inches to 0.275 inches, the diameter “D” can be between 0.450 inches and 0.550 inches, in accordance with some embodiments. 
     Furthermore, the volume of the gas expansion chamber  605 , in some examples, can be at least partially defined by the position of the insert  220  relative to the distal end of the barrel  115 . In more detail, depending on the position of the insert  220  relative to the distal end of the barrel  115 , the volume of the gas expansion chamber  605  may increase or decrease, thereby affecting the level of reduction in pressure, temperature, and/or velocity of combustion gas therein. As can be seen, the insert  220  can be a distance “X” from the end of the barrel  115 . In one example, distance “X” can be from less than 0.375 inches to 0.500 inches or greater. In such examples, the gas expansion chamber  605  can have a volume of approximately 0.140 to 0.185 cubic inches or more, depending on a given application. The gas expansion chamber  605 , in some examples, can have an axial length “C” in a range from 0.200 inches to 0.300 inches, depending on the position of the insert  220  relative to the barrel  115 . A gas expansion chamber  605  with an axial length “C” less than 0.2 inches or greater than 0.3 inches is acceptable in some embodiments. In some embodiments, the gas expansion chamber  605 , has a diameter “D” that is at least one and a half, two, three, or four times the diameter “B” of the bore  310  of the barrel  115 . 
     In addition, the location of the insert  220  relative to the barrel  115  can also affect the accuracy of the projectile  15  fired from the firearm  100 . For instance, in one example, an opening of the internal bore  510  of the insert  220  can be positioned an axial distance “X” from the barrel  115 . Distance “X”, in some examples, is sized such that part of the projectile  15  enters the bore  510  of the insert  220  before the projectile  15  fully exits the bore of the barrel  115 . Stated differently, the axial length X of the gas expansion chamber  605  is less than the axial length of the projectile  15 , in accordance with some embodiments. For example, projectiles of some 5.56×45 mm ammunition have an axial length from 0.750 to 0.940 inch. Thus, the projectile  15  bridges the gap (of axial length “X”) between the insert  220  and the distal end  115   a  of the barrel  115 . This can be particularly noteworthy because the insert  220  can enable movement of the projectile  15 , so that the projectile maintains its current trajectory as the projectile  15  passes through the gas expansion chamber  605 . In particular, the axial position of the insert  220  relative to the barrel  115  can prevent combustion gases from laterally affecting the flight of the projectile  15 . Otherwise, such forces can cause the projectile  15  to move off its intend path, and thereby reduce the accuracy of the firearm  100 . Thus, in some examples, the manner in which the insert  220  is attached to the gas block  205  can prevent the combustion gases within the gas expansion chamber  605  from surrounding or otherwise engulfing the projectile  15  as the projectile  15  moves through the gas expansion chamber  605 . 
     The gas expansion chamber  605  is also in fluid communication with the gas block port  420  of the gas block  205 . Specifically, the gas block port  520  extends between the gas expansion chamber  605  and the gas valve opening  935 , in accordance with some embodiments. Accordingly, the gas block port  420  is in direct communication with the gas expansion chamber  605 . As shown in  FIG. 7 , for example, the gas block port  420  can be located adjacent to the proximal end of the gas expansion chamber  605 , which is defined by the distal end  115   a  of the barrel  115 . In such a configuration, the gas block port  420  can receive expanded combustion gases at a pressure sufficient to operate the gas operating system  120  and without reducing the service life of the firearm. In some other examples, the gas block port  420  can also be in the middle or adjacent the distal end of the gas expansion chamber  605 , depending on a given application. Numerous configurations of the gas expansion chamber  605  will be apparent in light of the present disclosure. 
     Additional Gas Operating System Configurations 
       FIG. 8A  is a perspective view of a gas operating system  800  for a firearm  100 , and includes the insert  820  (not visible in  FIG. 8A ) within the gas block  810 , in accordance with another embodiment of the present disclosure.  FIG. 8B  is a cross-sectional view of the gas operating system  800  shown in  FIG. 8A . The gas operating system  800  shown in  FIGS. 8A-8B  can achieve an overall shorter firearm length without affecting performance and/or service life of the firearm  100 , in accordance with an embodiment of the present disclosure. In one example, the operating system  800  includes a barrel  805  attached to a gas block  810  in a similar fashion as described herein in relation to the gas operating system  120 . The gas block  810  further includes an insert  820  disposed therein, such that the distal end of the gas block  810  is the distal end of the firearm  100 . As shown in  FIG. 8B , for example, the insert  820  can be installed within the lower cylinder  812  of the gas block  810 . The proximal end  820   a  of the insert  820  can be open, such that the distal end  805   a  of the barrel  805  defines the proximal end of the gas expansion chamber  825 . For example, the insert  820  has a cup-like shape with a generally cylindrical insert body  826  that defines gas chamber  825  and extending axially to the proximal end  820   a  of the insert  820 . The generally cylindrical body  826  connects to the distal base  827  of the insert that defines pathway  822  distally of the gas expansion chamber  825  and through which the projectile exits the firearm. In some such embodiments, the proximal end  820   a  of the insert  820  abuts the distal end  805   a  of the barrel  805 , but this is not required in all embodiments. 
     In another example, the proximal end  820   a  of the insert  820  can be closed except for an entrance opening (not shown) to allow the projectile to move from the bore  815  to the air chamber  825 . In some such embodiments, for example, the insert  820  defines the air chamber  825  as an open region positioned axially between a proximal end portion (e.g., a proximal wall (not shown)) and the insert base  827 , where the proximal end may abut the distal end  805   a  of the barrel  805 . 
     As shown in  FIG. 8B , for example, the gas port  830  extends from the upper cylinder  813  to the lower cylinder  812  and through the insert  820  (e.g., through insert body  826 ) so that the air chamber  825  is in fluid communication with the valve  835  to operate the piston  840 . The gas block  810  also includes an exit aperture  845 , through which the projectile passes through to exit the firearm  100 . In some embodiments, the exit aperture  845  can have a frustoconical shape that increases in diameter as it extends axially from the internal pathway  822  to the distal end  810   a  of the gas block  810 . Numerous configurations and variations will be apparent in light of the present disclosure. 
     As shown in  FIG. 8B , for example, the insert  820  is housed completely within the lower cylinder  812  of the gas block  810  of the gas operating system  800 . Thus, the diameter of the gas expansion chamber  825  is the inner diameter of the insert  820  along the gas expansion chamber  825 . Together, the distal end  805  of the barrel  805 , the lower cylinder  812  of the gas block  810 , and the insert  820  define an air chamber  825  that functions like some embodiments of air chamber  605  discussed above. In the example shown in  FIG. 8B , an inside  821  of a distal end portion  827  of the insert  820  defines a bore or pathway  822  through which the projectile travels to exit the gas expansion chamber  825 . The inside  821  of the distal end portion  827 , in some examples, can be a flat surface perpendicular to the bore axis  806 . In such instances, the insert  820  can define the gas expansion chamber  825  having a uniform cross-sectional size and shape and that extends from the distal end  805   a  of the barrel  805  to the face at the inside  821  of the distal end portion  827 . In other examples, the inside  821  can extend proximally into the gas expansion chamber  825 , such as shown in  FIG. 8B . When the inside  821  of the distal end portion  827  extends into the gas expansion chamber  825 , it reduces the effective volume of the gas expansion chamber  825 . However, such a protrusion axially into the gas expansion chamber  825  enables the projectile to enter the internal pathway  822  of the insert  820  before exiting the bore  815  of the barrel  805 , and thus ensuring that the projectile maintains its intended path of travel. In some embodiments, the inside  821  of the distal end portion  827  can include one or more surfaces that are angled, curved, or otherwise inclined to the bore axis  806  and that promote turbulent flow of combustion gases within the gas expansion chamber  825 , similar to control surfaces  515  at the proximal end of insert  220  discussed above. In such cases, the gas expansion chamber  825  can have a non-uniform cross-sectional shape. 
       FIG. 9A  is a perspective view of a gas operating system  900  for a firearm  100 , in which the gas expansion chamber  925  is defined in the barrel  905 , in accordance with another embodiment of the present disclosure.  FIG. 9B  is a cross-sectional view of the gas operating system  900  shown in  FIG. 9A . In this one example, the gas operating system  900  includes a barrel  905 , a gas block  910 , and an insert  915 . In some embodiments, the barrel  905  extends through and distally beyond the distal end  910   a  of the gas block  910 . In some such configurations, the barrel  905  can receive the insert  915  rather than attaching the insert  902  to the gas block  910 . Additionally, the gas expansion chamber  925  is defined in the distal end portion  906  of the barrel  905 . For example, the gas expansion chamber  925  is defined distally of and adjacent to the rifled bore  920 , rather than being defined in the gas block  910 . Integrating the gas expansion chamber  925  into the barrel  905  enables the gas block  910  to be positioned concentric with the distal end portion  906  of the barrel  905  rather than adjacent thereto. When configured in this manner, the gas block  910  does not increase the overall length of the firearm  100 . In addition, the barrel  905  defines a gas port  930  extending between and fluidly connecting the gas expansion chamber  925  and a valve opening  935  in the gas block  910 . Together, the gas port  930  and the valve opening  935  provide fluid communication between the gas expansion chamber  925  and the valve  940  to operate piston  945 . 
       FIG. 10A  is a perspective view of a gas operating system  1000  for a firearm  100 , in which the insert  1015  is external to the barrel  1005 , in accordance with another embodiment of the present disclosure.  FIG. 10B  is a cross-sectional view of the gas operating system  1000  shown in  FIG. 10A . In some other examples, the gas block  1010  and the insert  1015  can be attached to an exterior surface of the barrel  1005  to further reduce the overall length of the firearm  100 . For example, the gas operating system  1000  includes a barrel  1005  with a gas block  1010  installed thereon, such that the block  1010  surrounds a portion of an exterior surface (or circumference) of the barrel  1005 . 
     As shown in  FIG. 10B , for example, with the gas block  1010  positioned on the barrel  1005 , the barrel  1005  extends through and distally beyond the gas block  1010  to enable the barrel to receive the insert  1015 . The insert  1015  can also be installed onto the outer (or exterior) surface of the barrel  1005 , thereby reducing the distance the insert  1015  extends beyond the barrel  1005 . The barrel  1005  defines a rifled bore  1020  and a gas expansion chamber  1025  adjacent to the distal end of the rifled bore  1020 . Note that the gas expansion chamber  1025  is defined in the distal end portion  1006  of the barrel  1005 , such that the gas expansion chamber  1025  is defined by the internal geometry of the barrel  1005  rather than a combination of the barrel  1005 , the gas block  1010 , and/or the insert  1015 . In addition, the barrel  1005  further defines a gas port  1030  in fluid communication with a valve opening  1035  of the gas block  1010 , where the gas port  1030  is located at the gas expansion chamber  1025  distally of the rifled portion of the bore  1020 . In some such embodiments, the diameter of the gas expansion chamber  1025  is slightly greater than that of the rifled portion of the bore  1020 . For example, the diameter of the gas expansion chamber  1025  is sized so that the projectile does not contact the barrel beyond the rifled portion. Distally of the gas expansion chamber  1025 , the barrel  1005  may define a greater diameter to allow expansion of gases exiting the barrel  1020 . When gases are permitted to expand to a greater extent upon leaving the bore  1020 , such as shown in  FIG. 10B , the gas expansion chamber  1025  may have a smaller diameter and/or volume so that gas pressure received in the valve effectively cycles the action of the firearm, as will be appreciated. The gas port  1030  is substantially aligned with the valve opening  1035  such that, together, the gas ports  1030  and valve opening  1035  enable the gas expansion chamber  1025  to be in fluid communication with the valve  1040  to operate piston  1045 . In some embodiments, the gas block  1010  defines a gas port  1012  between the gas port  1030  of the barrel and the valve opening  1035 , such as shown in  FIG. 10B . Numerous variations and configurations will be apparent in light of the present disclosure. 
       FIG. 11A  is a perspective view of a gas operating system  1100  for a firearm  100 , where the gas operating system lacks a removable insert, in accordance with another embodiment of the present disclosure.  FIG. 11B  is a cross-sectional view of the gas operating system  1100  shown in  FIG. 11A . In some examples, the gas operating system  1100  does not include a separate and removable insert that defines a gas expansion chamber and/or that enhances performance of the firearm  100  (e.g., by reducing muzzle flash). Rather, in some examples, the insert or its equivalent structure is defined in the barrel  1105  and is monolithic with the barrel  1105 . For example, gas operating system  1100  includes a barrel  1105  with a gas block  1110  disposed thereon. 
     As shown in  FIG. 11B , for example, the gas block  1110  is concentric with the barrel  1105 , such that the block  1110  does not extend beyond the muzzle-end  1105   a  of the barrel  1105 . The gas port  1130  and gas block port  1135  provide fluid communication between the gas expansion chamber  1120  and the valve  1140  to operate the piston  1145 . In addition, the barrel  1105  includes a bore  1115  and a gas expansion chamber  1120  integrated within the barrel  1005  and distally adjacent to a rifled portion  1115   a  of the bore  1115 . As shown in  FIG. 11B , for example, the gas expansion chamber  1120  has a greater diameter than the bore  1115 . The bore through distal end portion  1106  of the barrel  1105  at points distal of the gas expansion chamber  1120  can have a diameter that is greater than that of the gas expansion chamber, such as along the flash suppressing feature  1125 . In some embodiments, the barrel  1105  can optionally include a flash-suppressing feature  1125  integrated into the distal end portion  1106  of the barrel  1105 , the flash-suppressing feature  1125  configured to reduce muzzle flash resulting from burning propellant, for example. In some embodiments, the barrel  1105  can be further configured to receive other components, such as a silencer or suppressor, to enhance firearm performance. For example, the outside surface of the distal end portion  1106  of the barrel is threaded to receive an attachment, such as along all or part of the flash suppressing feature  1125 . Numerous other insert configurations will be apparent in light of the present disclosure. 
     Further Considerations 
       FIGS. 12A-12E  are photographs showing stages of erosion of a gas port  1250  in a conventional operating system of a firearm. The gas port in  FIGS. 12A-12E  is located in the rifled portion of the barrel. The top of each photograph is the down-bore direction (i.e., towards the distal end of the barrel). As previously described, gas port erosion is a particular concern for short-barreled firearms that fire rifle-caliber ammunition, because the gas port  1250  is located near the chamber of the barrel due to the reduced length of the barrel. The reduced distance between the chamber and the gas port causes high pressure, temperature, and/or velocity combustion gases to enter the gas port and thereby rapidly erode the gas port. 
       FIG. 12A  is a photograph looking into the gas port  1250  of a conventional short-barreled firearm as initially machined through the wall of the barrel. In  FIG. 12A , the gas port is shown as a dark circle in the center of the lighter-colored region of the inside surface  1260  of the barrel. Note that the gas port  1250  is well-defined with a uniform circular shape, and the inside surface  1260  of the barrel is relatively smooth. After a short period of firearm use, erosion  1255  of the gas port  1250  begins to occur. This erosion  1255  can begin to occur after firing as few as 1,200 to 2,400 rounds from the firearm. As can be seen in  FIGS. 12B and 12C , the gas port  1250  no longer has a uniform circular shape, but appears elongated towards the down-barrel side of the gas port  1250 . In addition, the inside surface  1260  of the barrel includes grooves and other surface defects  1270  caused by gases and particles moving at high temperature, high pressure, and high velocity across and into the gas port  1250 . 
     Further erosion  1255  of the gas port  1250  occurs with additional use. For example, as shown in  FIGS. 12D-12F , after firing 3,600 to 6,000 rounds, the gas port  1250  is further eroded. This erosion  1255  adversely affects firearm performance and accuracy, as previously described herein. In  FIGS. 12D-12F , the opening to the gas port  1250  is significantly deformed, such that the gas port no longer has its initial machined diameter. In addition to erosion  1255  on the down-barrel side of the gas port  1250 , a recess is defined around the gas port  1250  that provides a larger effective entrance to the gas port  1250 , such as shown in  FIG. 12F . The enlarged entrance to the gas port can allow more gases than needed (or desired) to flow into the gas operating system, thereby causing the system to operate faster than designed. The faster movement of system components can cause increased wear and/or damage to the components as the forces applied to individual components can increase. In addition, significant amount of surface material has been removed from the inside surface  1260  of the barrel, such that the interface between the projectile and the barrel has been impaired, thereby adversely affecting the flight of the projectile and/or accuracy of the firearm. 
     In contrast, the gas operating system of the present disclosure does not experience similar erosion. Rather, the gas port experiences very little (if any) erosion of the gas port after significant use of the firearm.  FIGS. 13A-13B  are example photographs of a gas port  1310  after firing 5,000 rounds, where the gas port  1310  is located in the gas expansion chamber.  FIG. 13A  shows the gas port  1310  as viewed at an angle using a borescope.  FIG. 13B  is a view looking into the gas port  1310  and also shows the smaller valve port  1320 . In both  FIGS. 13A-13B  the entrance to the gas port  1310  shows little or no erosion. Also, the shape of the gas port  1310  is substantially maintained. This is particularly noticeable in  FIG. 13B , in which the photograph illustrates that the gas port  1310  maintains a substantially uniform diameter as initially machined. 
     EXAMPLE EMBODIMENTS 
     The following examples pertain to further embodiments, from which numerous permutations and configurations will be apparent. 
     Example 1 is a gas operating system for a firearm, the system comprising a barrel extending from a proximal end to a distal end, the barrel defining a bore extending therethrough along a bore axis, the bore including a rifled bore portion with a first bore diameter; and a gas block attached to a distal end portion of the barrel; wherein the gas operating system defines a gas expansion chamber located distally of the rifled bore portion, the gas expansion chamber in fluid communication with the bore and the gas block and having a chamber diameter greater than the first bore diameter. 
     Example 2 includes the subject matter of Example 1, wherein the gas expansion chamber is defined within the gas block. 
     Example 3 includes the subject matter of Example 2 and further comprises an insert attached to the gas block, the insert positioned to define at least one end of the gas expansion chamber and aligned with the bore axis such that a projectile moves through the rifled bore portion, the gas expansion chamber, and the insert during a firing cycle. 
     Example 4 includes the subject matter of Example 3, wherein the insert receives part of the projectile before the projectile completely exits the rifled portion of the bore during a firing cycle of the firearm. 
     Example 5 includes the subject matter of Example 3 or 4, wherein the insert is disposed completely within the gas block and adjacent to the distal end the barrel. 
     Example 6 includes the subject matter of any of Examples 3-5, wherein the insert has a proximal end portion defining one or more control surfaces configured to re-direct a flow of combustion gases during a firing cycle of the firearm. 
     Example 7 includes the subject matter of any of Examples 3-4 and 6, wherein the insert is further configured to reduce muzzle flash during a firing cycle of the firearm. 
     Example 8 includes the subject matter of any of Examples 3-4 and 6-8, wherein the insert is further configured to receive at least one additional firearm component. 
     Example 9 includes the subject matter of Example 1, wherein the gas block includes a piston, and the gas block defines a gas block port located adjacent to a distal end of the rifled bore portion, wherein the gas block port is in direct communication with the gas expansion chamber and wherein the piston is configured to move in response to receiving gases from the barrel via the gas expansion chamber and the gas block port. 
     Example 10 includes the subject matter of Example 9, wherein a distal end portion of the barrel defines the gas expansion chamber located distally of the rifled bore portion, and defines a gas port in communication with the gas expansion chamber and the gas block port. 
     Example 11 includes the subject matter of Example 10, wherein the barrel comprises a second bore distally adjacent the rifled bore portion, the second bore defining the gas expansion chamber and having a second bore diameter greater than the first bore diameter. 
     Example 12 includes the subject matter of Example 11, wherein the distal end portion of the barrel further defines a third bore located distally of the second bore, the third bore having a third bore diameter greater than the second bore diameter. 
     Example 13 includes the subject matter of any of Examples 1-12, wherein the barrel has a length from 5 to 9 inches. 
     Example 14 includes the subject matter of any of Examples 1-13, wherein the firearm is chambered for 5.56×45 mm ammunition. 
     Example 15 includes the subject matter of Example 14, wherein the gas expansion chamber has an axial length less than an axial length of a projectile of the ammunition. 
     Example 16 includes the subject matter of any of Examples 1-15, wherein the chamber diameter at least twice the bore diameter. 
     Example 17 is a gas operating system for a firearm, the system comprising a gas block defining a first cylinder, a second cylinder in fluid communication with the first cylinder, and a gas port in direct communication with the first cylinder; and a barrel extending longitudinally and defining a bore with a bore diameter, the barrel having a distal end portion with a distal barrel end, wherein the distal end portion of the barrel is received in the first cylinder with the distal barrel end positioned proximally of the gas port; wherein the first cylinder is in fluid communication with the bore via the gas port. 
     Example 18 includes the subject matter of Example 17, wherein the distal barrel end and part of the first cylinder define a gas expansion chamber in fluid communication with the gas port. 
     Example 19 includes the subject matter of Example 18 or 19 and further comprises an insert installed in a distal portion of the first cylinder, the insert axially spaced from the distal barrel end to define a gas expansion chamber therebetween, wherein the chamber is in direct fluid communication with the gas port. 
     Example 20 includes the subject matter of Example 19, wherein the insert includes a proximal end portion that defines a control surface configured to re-direct a flow of combustion gases within the gas expansion chamber. 
     Example 21 includes the subject matter of Examples 19 or 20, wherein the insert is configured to reduce muzzle flash. 
     Example 22 includes the subject matter of Example 18 and further comprises an insert disposed within the first cylinder distally of the distal barrel end, the insert configured and positioned to define a gas expansion chamber adjacent the distal barrel end, wherein the gas expansion chamber is in direct fluid communication with the gas port and the gas expansion chamber has a chamber diameter greater than the bore diameter. 
     Example 23 includes the subject matter of Example 22, wherein the insert is disposed within the gas block such that an opening within the gas block forms a muzzle-end of the firearm. 
     Example 24 includes the subject matter of Examples 22 or 23, wherein a distal end portion of the insert extends proximally into the gas expansion chamber and defines a projectile pathway therethrough. 
     Example 25 includes the subject matter of Example 24, wherein an entrance to the projectile pathway is less than 0.75 inch from the distal barrel end. 
     Example 26 includes the subject matter of any of Examples 18-25, wherein the barrel has a length from 5 to 9 inches. 
     Example 27 includes the subject matter of any of Examples 18-26, wherein the chamber diameter is at least twice the bore diameter. 
     Example 28 includes the subject matter of any of Examples 18-27, wherein the gas expansion chamber has an axial length of less than one inch. 
     Example 29 includes the subject matter of Example 28, wherein the axial length is less than 0.75 inch. 
     Example 30 includes the subject matter of Example 28, wherein the axial length is less than 0.5 inch. 
     Example 31 includes the subject matter of any of Examples 18-30 and further comprises a gas valve and a piston in the second cylinder, the piston configured to move in response to receiving gases from the bore via the gas port. 
     The foregoing description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto.