Abstract:
A bolt catch assembly including a bolt engagement leg, a sliding member engagement leg, a contact member and an arrangement for enabling the bolt catch assembly to pivot about a pivot axis. The bolt engagement leg is configured for engaging a bolt of a firearm to hold the bolt in an open position. The sliding member engagement leg is connected to the bolt engagement leg. The contact member resiliently mounted on the sliding member engagement leg. The contact member is configured for being displaced between a static position and a displaced position whereby a catch engagement force is generated in response to the contact member being moved from the static position toward the displaced position. The arrangement for enabling the bolt catch assembly to pivot is disposed between the bolt engagement leg and the sliding member engagement leg thereby enabling the bolt engagement leg and the sliding member engagement leg to be pivoted in unison about the pivot axis of the bolt catch assembly.

Description:
CROSS-REFERNCE TO RELATED APPLICATION  
       [0001]     This is a Divisional Utility patent application to co-pending United States Utility patent application having Ser. No. 09/734,279 filed on Dec. 11, 2000. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The disclosures herein relate generally to firearms, and more particularly to firearm upper receivers with belt-feed capability.  
         [0003]     Many firearms, such as assault rifles, that are commonly used in military situations are not designed by their manufacturer for use with belt-feed ammunition. Typically, such firearms are designed by their manufacturer for receiving ammunition from an ammunition magazine. The AR-15 family of firearms, including the M-16 type firearms, illustrate examples of assault rifles that are designed by their manufacturer to receive ammunition exclusively from an ammunition magazine. M-16 type firearms are a military version of the AR-15 family of firearms capable of operating in a fully automatic mode. M-16 type firearms have been manufactured by companies including, but not limited to Colt Manufacturing Company, the ArmaLite Division of Fairchild Aircraft and Engine Company, BushMaster Firearms Incorporated and Fabrique Nationale. A standard ammunition magazine for M-16 type firearms holds approximately 30 rounds of ammunition. The versatility of firearms that are intended for use in military situations and that are designed for receiving ammunition exclusively from an ammunition magazine is significantly limited.  
         [0004]     Some firearms, such as M-16 type firearms, may be operated in a fully automatic mode. When being operated in the fully automatic mode, firing of a round of ammunition automatically facilitates ejection of each spent round from the firing chamber and chambering of a new round into the firing chamber. As long as the trigger of such as firearm is depressed, the firearm will continue to fire until all of the ammunition is depleted.  
         [0005]     Due to the attainable firing rate of firearms operated in a fully automatic mode and the limited ammunition capacity of standard ammunition magazines, the use of ammunition magazines with such firearms results in a significant amount of down-time of the firearm for allowing a depleted magazine to be replaced with a full ammunition magazine. Most automatic firearms are capable of firing ammunition at a rate of 150 rounds or more per minute. At a firing rate of 150 rounds per minute, a 30 round ammunition magazine can be depleted of ammunition in as little as about 12 seconds of continuous firing.  
         [0006]     In many situations, such as in military combat, a high-capacity ammunition delivery system such as a belt-feed system is preferred over an ammunition magazine. A typical ammunition belt for a belt-feed system holds  200  or more rounds of ammunition. At a firing rate of 150 rounds per minute, a 200 round ammunition belt can be depleted in as little as about 80 seconds. Accordingly, for a given firearm design, the minimum time to depletion of a 200 round ammunition belt is as much as about 7 times greater than that of a 30 round ammunition magazine. As a result of the increased time to depletion, belt-feed ammunition systems are preferred in many military situations.  
         [0007]     Attempts have been made to increase the versatility of magazine-fed firearms by modifying them to accept belt-feed ammunition. The CAR-15 heavy assault rifle model M2, developed by Colt Manufacturing Company, illustrates an example of such a modified firearm. The ArmaLite Division of the Fairchild Engine and Airplane Corporation also developed such a modified firearm for receiving magazine-fed and belt-feed ammunition.  
         [0008]     To date, magazine-fed firearms that have been modified to accept belt-feed ammunition, including those discussed above, have required modification to an upper receiver assembly and a lower receiver assembly of the firearm. Facilitating modifications to the upper and to the lower receiver assemblies is costly. Furthermore, the lower receiver assembly of many firearms, such as M-16 type firearms, is the registerable portion of the firearm that carries a serial number for enabling compliance with registration requirements of the United States Bureau of Alcohol, Tobacco &amp; Firearms. As a result of the lower receiver assembly being the portion of the firearm that is registerable, it can only be modified legally by a licensed firearm manufacturer.  
         [0009]     The bolt carrier group of many automatic firearms, such as M-16 type firearms, are energized using pressure generated by the combustion of powder in a cartridge. Such firearms are considered to be gas energized. In such firearms, it is typical for combustion gas to be routed from the barrel to the receiver assembly that carries the bolt carrier group (referred to herein as the bolt-carrying receiver). In this manner, pressure associated with the combustion gas is used to supply the energy needed for facilitating ejection of a spent cartridge from the firing chamber and feeding of a new round of ammunition into the firing chamber. Accordingly, the bolt carrier groups of types of firearms are gas driven as well as gas energized.  
         [0010]     The routing of the combustion gas to the bolt-carrying receiver results in several adverse situations. One adverse situation is that over time, deposits from the combustion gas are formed inside the bolt-carrying receiver. Such deposits adversely affect operation of the firearm and, in some cases, prevent its operation until the bolt-carrying receiver is cleaned. Another adverse situation is that the combustion gases are vented into the general area of an operator&#39;s face, impairing the operator&#39;s sight and respiration.  
         [0011]     Accordingly, a receiver system that enables the shortcomings associated with conventional firearms would be useful.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  is a side view illustrating an embodiment of a firearm having an ammunition belt attached to an upper receiver assembly.  
         [0013]      FIG. 1B  is a side view of the firearm of  FIG. 1A  having an ammunition magazine attached to a lower receiver assembly, and the ammunition belt detached from the upper receiver assembly.  
         [0014]      FIG. 1C  is a side view illustrating an embodiment of a trigger group in the lower receiver assembly of the firearm of  FIG. 1A .  
         [0015]      FIGS. 2A-2H  are fragmentary side views illustrating an embodiment of an operational cycle of the firearm of  FIG. 1B  with the ammunition being supplied from an ammunition magazine.  
         [0016]      FIG. 3A  is a side view illustrating an embodiment of an upper receiver assembly having a piston tube assembly and a barrel assembly attached thereto.  
         [0017]      FIG. 3B  is a perspective view of the upper receiver assembly, the piston tube assembly and barrel assembly depicted in  FIG. 3A .  
         [0018]      FIG. 4  is side view illustrating the barrel assembly depicted in  FIG. 3A .  
         [0019]      FIGS. 5A and 5B  are cross-sectional views illustrating an embodiment of a firearm having an adjustable gas regulator coupled to a piston tube assembly for displacing a tappet assembly, with an operating rod of the piston tube assembly being in a static position and a displaced position, respectively.  
         [0020]      FIGS. 6A and 6B  are side views illustrating an embodiment of a tappet assembly in relation to the displaced position and the static position, respectively, of the operating rod depicted in  FIGS. 5A and 5B .  
         [0021]      FIG. 7  is a cross-sectional view taken along the line  7 - 7  in  FIG. 6A .  
         [0022]      FIG. 8  is a partial top view illustrating an upper receiver assembly as disclosed herein.  
         [0023]      FIG. 9  is a cross-sectional view taken along the line  9 - 9  in  FIG. 8 .  
         [0024]      FIG. 10  is a cross-sectional view taken along the line  10 - 10  in  FIG. 8 .  
         [0025]      FIG. 11  is a partial perspective view illustrating an embodiment of a mechanism for rotating a bolt, with the bolt being depicted in an unlocked position.  
         [0026]      FIG. 12  is a partial top perspective view of the mechanism depicted in  FIG. 11 , with the bolt being depicted in a locked position.  
         [0027]      FIG. 13  is an exploded perspective view illustrating embodiments of a bolt, a firing pin, and cam pin.  
         [0028]      FIG. 14  is a perspective view illustrating another embodiment of a mechanism for rotating a bolt.  
         [0029]      FIG. 15  is a partial side view of the mechanism depicted in  FIG. 14  mounted in an upper receiver body, with the bolt being depicted in the unlocked position.  
         [0030]      FIG. 16  is a partial side view of the mechanism depicted in  FIG. 14  mounted in an upper receiver body, with the bolt being depicted in the locked position.  
         [0031]      FIG. 17  is a perspective view illustrating an embodiment of a bolt carrier of the mechanism depicted in  FIG. 14 .  
         [0032]      FIG. 18  is a partial perspective view illustrating an embodiment of an ammunition belt feeding assembly.  
         [0033]      FIG. 19  is a top view illustrating an embodiment of a top cover of the ammunition belt feeding assembly depicted in  FIG. 18 .  
         [0034]      FIG. 20  is a perspective view illustrating an embodiment of a feed tray of the ammunition belt feeding assembly depicted in  FIG. 18 .  
         [0035]      FIGS. 21A and 21B  are diagrammatic views illustrating an embodiment of a lever-type ammunition belt feeding mechanism with a cam lever in a static position and a displaced position, respectively.  
         [0036]      FIG. 22  is a plan view illustrating an embodiment of a feed link of the ammunition belt feeding mechanism depicted in  FIGS. 21A and 21B .  
         [0037]      FIG. 23  is a plan view illustrating an embodiment of a first slide member of the ammunition belt feeding mechanism depicted in  FIGS. 21A and 21B .  
         [0038]      FIG. 24  is a plan view illustrating an embodiment of a second slide member of the ammunition belt feeding mechanism depicted in  FIGS. 21A and 21B .  
         [0039]      FIGS. 25A-25E  are diagrammatic views illustrating an embodiment of an operational cycle of the ammunition belt feeding mechanism depicted in  FIGS. 21A and 21B .  
         [0040]      FIG. 26  is a diagrammatic view illustrating an embodiment of a sprocket-type ammunition belt feeding mechanism.  
         [0041]      FIG. 27  is an exploded perspective view illustrating an embodiment of a drive shaft assembly of the sprocket-type ammunition belt feeding mechanism depicted in  FIG. 26 .  
         [0042]      FIGS. 28A-28C  are diagrammatic views illustrating an embodiment of an operational cycle of the ammunition belt feeding mechanism depicted in  FIG. 26 .  
     
    
     DETAILED DESCRIPTION  
       [0043]     An embodiment of a firearm  10  including an upper receiver assembly  12  and having an ammunition belt  14  attached to the upper receiver assembly  12  is depicted in  FIG. 1A . The firearm  10  is depicted in  FIG. 1B  having an ammunition magazine  16  attached to a lower receiver assembly  18  of the firearm  10 . As depicted in  FIG. 1C , the lower receiver assembly  18  includes a lower receiver body  19  having a trigger group  20  mounted thereon. The trigger group  20  comprises a trigger  22 , a hammer  24 , a disconnect  26 , and an automatic sear  28 .  
         [0044]     A lower receiver assembly from an M- 16  type firearm illustrates an example of the lower receiver assembly  18 . M-16 type firearms are manufacturer configured for receiving ammunition exclusively from an ammunition magazine attached to their lower receiver assembly. The upper and lower receiver assemblies of an unmodified M-16 type firearm illustrate examples of as-manufactured original equipment manufacturer (OEM) upper and lower receiver assemblies.  
         [0045]     It is advantageous to enable a firearm configured by its manufacturer for receiving ammunition exclusively from an ammunition magazine to also receive ammunition from an ammunition belt. For firearms having a registerable lower receiver assembly, it is particularly advantageous for the an upper receiver assembly capable of supplying ammunition from an ammunition belt to be mountable on an unmodified lower receiver assembly. In this manner, such an upper receiver assembly may be legally fitted to the registerable lower receiver assembly by parties other than the manufacturer.  
         [0046]     An embodiment of an operational cycle of the firearm  10  for ammunition supplied from the magazine  16  is depicted in  FIGS. 2A-2H . When the firearm  10  has a selector switch (not depicted) set for semi-automatic fire, the operational cycle begins with a chambered round  30  in a firing chamber  31  and the hammer  24  in a cocked position H 1  with a lower hammer notch  24   a  engaged with a trigger sear  22   a , as depicted in  FIG. 2A . Each round of ammunition includes a cartridge and a bullet. The chambered round  30  includes a bullet  30   a  that is projected down a barrel  33  when the chambered round  30  is fired.  
         [0047]     As the trigger  22  is pulled from a ready position R,  FIG. 2A , to a firing position F,  FIG. 2B , the hammer  24  is released and rotates forward, striking a firing pin  32  thereby causing the chambered round  30  to be fired and a bullet  30   a ,  FIG. 2A , to be projected down a barrel  33 . The firing pin  32  is mounted on a bolt  34  and the bolt  34  is mounted on a bolt carrier  36 . The bolt  34  and the bolt carrier comprise a bolt carrier group. As the bullet  30   a  travels down the barrel  33 , combustion gas  38  creates pressure in the barrel  33  between the bullet  30   a  and the chambered round  30 ,  FIG. 2B . The pressure associated with the combustion gas  38  facilitates ejection of the chambered round  30  and chambering of an unfired round  40  via a conventional gas-driven bolt actuating technique, such as that used in Colt M-16 type firearms, or an embodiment of a piston-driven bolt actuating technique as disclosed herein.  
         [0048]     Regardless of the bolt actuating technique used, firing of the chambered round  30  results in the bolt  34  and the bolt carrier  36  being moved in a rearward direction away from the barrel  33  from a closed position C,  FIG. 2C , toward an open position O,  FIG. 2D . Accordingly, the bolt carrier group and all of its components are moved from the closed position C toward the open position O. In response to the bolt carrier  36  being moved in the rearward direction, the bolt  34  is rotated such that lugs of the bolt  34  are unlocked from corresponding lugs of a barrel extension. In this manner, the bolt  34  is free to move, as a component of the bolt carrier group, from the closed position C toward the open position O. As the bolt  34  and bolt carrier  36  move in the rearward direction, the chambered round  30  is withdrawn from the firing chamber and is ejected from the firearm  10  through an ejection port. The movement of the bolt carrier  36  in the rearward direction also returns the hammer  24  from a firing H 2 ,  FIG. 2B , to the cocked position H 1 ′,  FIG. 2D , with an upper hammer notch  24   b  engaged with a disconnect hook  26   b.    
         [0049]     The rearward movement of the bolt carrier  36 , and consequently the bolt  34 , is arrested by a buffer assembly  41 ,  FIG. 2C . The buffer assembly  41  includes an action spring  41   a  that is compressed by the bolt carrier  36  during its rearward movement. As depicted in  FIG. 2D , the compressed action spring  41   a  forces the bolt carrier group in a forward direction towards the closed position C, towards the barrel  33 . Upon moving forward toward the closed position C, the bolt  34  engages the unfired round  40  in the magazine  16  and thrusts the unfired round  40  into the firing chamber  31 ,  FIG. 2E . As the bolt carrier  36  and the bolt  34  continue to move towards the closed position C, the lugs of the bolt  34  enter the bolt extension of the barrel  33  and the bolt  34  engages a face of the barrel extension. An ejector pin is depressed against the unfired round  40  and an extractor snaps into an extracting groove of the unfired round  40 , facilitating ejection after the unfired round  40  is fired.  
         [0050]     While the bolt  34  is engaged with the face of the barrel extension, the bolt carrier  36  continues to move towards the closed position C. As the bolt carrier  36  continues to move in the forward direction toward the closed position C, the bolt  36  is rotated such that the lugs of the bolt  34  are locked relative to the lugs of the barrel extension. The bolt carrier group is said to be in the closed position C when the lugs of the bolt  34  are locked relative to the lugs of the barrel extension. Mechanisms and techniques for rotating the bolt  34  such the lugs can be locked and unlocked from the lugs of the barrel extension are disclosed below in greater detail.  
         [0051]     When the selector switch is set to the semi-automatic position, firing the unfired round  40  requires releasing and pulling the trigger  22  for each fired round. When the trigger is released, a trigger spring  22   c ,  FIG. 2E , causes the trigger  22  to move from the firing position F to the ready position R,  FIG. 2F . Releasing the trigger  22  also causes the upper hammer notch  24   b  to disengage from the disconnect hook  26   b . In this manner, the hammer  24  is released, allowing it to move to the cocked position H 1 ,  FIG. 2F , with the lower hammer notch  24   a  engaged with the trigger sear  22   a . The firearm is now ready to fire the unfired round  40 .  
         [0052]     Moving the selector switch (not depicted) to the automatic position permits the firearm to operate in a fully automatic mode. With the selector switch set in the automatic position,  FIG. 2G , a lower edge  28   a  of the automatic sear  28  engages a top outside hammer notch  24   c  during the rearward movement of the bolt carrier  36 . This action holds the hammer  24  in the automatic cocked position H″. During the forward movement of the bolt carrier  36 ,  FIG. 2H , the bolt carrier  36  strikes an upper edge  28   b  of the automatic sear  28 , releasing the automatic sear  28  from the hammer  24  thereby permitting the hammer  24  to strike the firing pin  32  and fire the unfired round  40 . In this manner, rounds of ammunition will be automatically fired, ejected and chambered until the trigger  22  is released or all of the rounds are used.  
         [0053]     As depicted in  FIGS. 3A and 3B , the upper receiver assembly  12  includes an upper receiver body  42 . A piston tube assembly  44  is attached to the upper receiver body  42 . The piston tube assembly  44  includes a piston tube  46  having a tappet assembly  47 ,  FIG. 3B , movably mounted thereon. The piston tube  46  includes a first end  46   a  that is mounted in a piston tube receptacle  48  of the upper receiver body  42 . A press pin  50  extends through the upper receiver body  42  and a corresponding hole in the piston tube  46 , securing the piston tube  46  in place relative to the upper receiver body  42 .  
         [0054]     The tappet assembly  47 ,  FIG. 3B , includes a yoke  47   a  that rides on the piston tube  46  and a tappet rod  47   b  attached to the yoke  47   a . The tappet rod  47   b  extends from the yoke  47   a  through the upper receiver body  42  into contact with a bolt carrier lug  36   a ,  FIG. 7  that is movably mounted on the upper receiver body  42 . The tappet rod  47   b  and the charging member  51  extend along substantially parallel longitudinal axes.  
         [0055]     A barrel assembly  52 ,  FIGS. 3-4 , is configured for being attached to the upper receiver assembly  12 . The barrel assembly  52  includes the  33  (discussed above in reference to  FIGS. 2A-2H ) and a gas block  56 ,  FIGS. 3A and 4 , attached to the barrel  33 . A pressure regulator  58 ,  FIGS. 3A and 4 , is mounted in the gas block  56 . A first end  33   a  of the barrel  33  is configured for being received in a barrel receptacle  60 ,  FIG. 3B , of the upper receiver body  42 . A nipple  58   a ,  FIG. 4 , of the pressure regulator  58  is configured for being received in a second end  46   b ,  FIG. 3A , of the piston tube  46 .  
         [0056]     As depicted in  FIG. 3B , the upper receiver assembly  12  includes a barrel retention mechanism  62  pivotally mounted thereon for securing the barrel assembly  52  to the upper receiver body  42 . The barrel retention mechanism  62  is biased by a spring  62   a  to a locked position L 1 . By depressing a release lever portion  62   b  of the barrel retention mechanism  62 , a pin extending through the upper receiver body  42  is disengaged from the barrel  33 , permitting the barrel  33  to be withdrawn from the barrel receptacle  60 .  
         [0057]     Referring to  FIGS. 5A and 5B , the piston tube assembly  44  includes an operating rod  64  movably mounted in a bore  46   a  of the piston tube  46 . A piston  66  is attached at a first end  64   a  of the operating rod  64 . The yoke  47   a  is attached to the operating rod  64  by a pin  68 . The pin  68  extends through the yoke  47   a  and the operating rod  64 . The piston tube  46  has opposing elongated slots  46   b  through which the pin  68  extends, allowing the yoke  47   a  and the operating rod  64  to move along the longitudinal axis of the piston tube  46 . A return spring  70  is captured in the bore  46   a  of the piston tube  46  between a second end  64   b  of the operating rod  64  and a closed end portion  46   c  of the piston tube  46 . The return spring  70  biases the operating rod  64  to a static position S.  
         [0058]     A passage  72  extends through the barrel  33  to the pressure regulator receptacle  56   a  of the gas block  56 . The pressure regulator  58  depicted in  FIGS. 5A and 5B  is an adjustable pressure regulator including a plurality of orifices  58   b  extending between an outer surface  58   c  and a gas communication passage  58   d  of the pressure regulator  58 . During operating of the firearm  10 , one of the orifices  58   b  is aligned with the passage  72 .  
         [0059]     When a chambered round of ammunition in the firearm  10  is fired,  FIG. 5B , a bullet  74  travels down the bore of the barrel  33 . Firing of the chambered round of ammunition produces combustion gases creating pressure in the bore of the barrel  33  between the bullet  74  and the cartridge of the fired round of ammunition. When the bullet travels past the passage  72 , a portion of the combustion gas travels through the passage  72  and the pressure regulator  58  into the bore  46   a  of the piston tube  46 . In doing so, a face of the piston  66  is exposed to pressure associated with the combustion gases. The pressure drives the piston  66 , and consequently the operating rod  64  from the static position S to a displaced position D, compressing the return spring  70 .  
         [0060]     One or more gas exhaust ports  76  are formed in the piston tube  46  adjacent to the displaced position D for venting the combustion gas to the ambient environment. Upon venting the combustion gases, the return spring  70  biases the piston  66  and operating rod  64  towards the static position S. A vent hole  78  may be provided in the piston tube for relieving movement-induced pressure behind the piston  66 .  
         [0061]     The pressure regulator  58  may be rotated for individually aligning a particular one of the orifices  58   b  with the passage  72 . By each of the orifices  58   b  being a different size, the amount of pressure exerted on the piston  66  can be selectively varied. In many situations, it will be advantageous to adjust the pressure that is exerted on the piston. For example, to maintain a desired level of performance of the firearm  10  as components of the firearm  10  wear, as the components become fouled from the combustion gas or when the firearm is used in different ambient environments, it is advantageous to be able to compensate for such situations. However, in some applications, the pressure regulator  58  may have only one orifice  58   b , resulting in the pressure regulator being non-adjustable. In the case of a non-adjustable pressure regulator, the size of the orifice  58   b  will be determined based on a compromise for intended and predicted conditions.  
         [0062]     As depicted in  FIGS. 6A and 6B , displacement of the operating rod  64  from the static position S to the displaced position D results in a corresponding displacement of the yoke  47   a . The tappet rod  47   b  is engaged with a bolt carrier lug  36   a  of the bolt carrier  36 . The bolt carrier lug  36   a  is constrained to forward and rearward movement in a bolt carrier lug channel  42   b ,  FIG. 7 , of the upper receiver body  42 . Accordingly, the displacement of the operating rod  64  also results in a corresponding displacement of the bolt carrier  36 . The displacement of the bolt carrier  36  that is associated with the displacement of the operating rod  64  is an initial displacement of the bolt carrier  36 . Due to inertia associated with the speed at which the operating rod  64  is displaced, the bolt carrier  36  continues to travel after the operating rod  64  reached its maximum displacement. Thus, the overall displacement of the bolt carrier  36  is greater than the displacement of the operating rod  64 . Accordingly, the upper receiver assembly is said to be gas energized and piston driven.  
         [0063]     Implementation of embodiments of the piston tube assembly  44  and tappet assembly  47  are advantageous. One advantage is that the piston tube assembly  44  and the tappet assembly  47  transfer the energy associated with the combustion gases more efficiently to the bolt carrier  36 . Because the piston  66  is mechanically coupled through the operating rod  64  and the tappet assembly to the bolt carrier  36 , the length over which the combustion gases must travel to build sufficient pressure to energize the bolt carrier  36  is significantly reduced. Accordingly, the length over which compression of the combustion gas occurs is significantly reduced. By reducing the length over which compression of the combustion gases occurs and by mechanically coupling the piston  66  to the bolt carrier  36 , the bolt  34  and the bolt carrier  36  are more efficiently moved from the closed position towards the open position.  
         [0064]     Another advantage associated with the piston tube assembly  44  and the tappet assembly  47  relates to fouling of the firearm associated with the combustion gases. Conventional gas driven bolt actuation mechanisms result in fouling of the upper and lower receiver assemblies of a firearm. Fouling of the firearm can result in degraded performance of the firearm and, if not timely addressed, malfunction of the firearm. Because embodiments of the piston tube assembly  44  and the tappet assembly  47  disclosed herein preclude the need to route combustion gases to the upper receiver assembly  12 , the potential for the combustion gases to foul of the upper receiver assembly  12  and the lower receiver assembly  18  is greatly reduced.  
         [0065]     The piston tube assembly  44  and the pressure regulator  58  are susceptible to being fouled by the combustion gases. However, when these components require cleaning, they may be quickly and easily detached from the upper receiver assembly  12  to facilitate cleaning. It is a significant advantage that when fouled, the piston tube assembly  44  and the pressure regulator  58  can be detached, cleaned and re-attached to the upper receiver assembly  18  in a timely manner. Furthermore, because the piston tube assembly  44  is a unitary assembly, it can be quickly and easily replaced. In situations such as military combat, it may be desirable and advantageous to replace the piston-tube assembly rather than clean it.  
         [0066]     Yet another advantage associated with embodiments of the piston tube assembly  44  disclosed herein is the location at which the combustion gases are vented. In some conventional firearms such as M-16 type firearms, during firing of the firearm, the combustion gases are vented from the firearm very close to the firearm operator&#39;s face. As a result, the vision and respiration of the operator may be impaired. Implementation of an embodiment of the piston tube assembly  44  disclosed herein results in the combustion gases being vented at a location that significantly reduces the potential for the vision and respiration of the operator to be impaired.  
         [0067]     The design of this piston tube assembly allows the tappet to contact a portion of the bolt carrier that is not directly in line with the piston. In this manner, a bipod mounting bracket may be fitted to the piston tube in a manner in which the bipod attachment does not hinder removal of the barrel. In conventional configurations, the bipod mounting bracket is attached to the barrel, thus making the barrel difficult to remove with the weapon supported on the bipod. Furthermore, this results in each barrel having the added weight of a bipod mounting bracket.  
         [0068]     Referring to  FIG. 7 , the tappet rod  47   b  engages a first surface  36   a ′ of the bolt carrier lug  36   a . The charging member  51  includes a charging member lug  51   a  that engages a second surface  36   a ″ of the bolt carrier lug  36   a . The charging member  51  includes flanges  51   b  that are each received by a respective groove  42   a  of the upper receiver body  42 , thus allowing the charging member  51  to be displaced relative to the upper receiver body  42 . The configuration and orientation of the bolt carrier lug  36   a , the tappet rod  47   b  and the charging member lug  51   a  permits the bolt carrier  36  to be manually displaced by pulling on a charging handle  51   c  of the charging member  51 .  
         [0069]     Referring to  FIGS. 8-10 , a bolt catch  80  is pivotally attached to the lower receiver body  19  at a pivot pin  81 . The bolt catch  80  includes an upper leg  80   a  and a lower leg  80   b . The pivot pin  81  is positioned between the upper leg  80   a  and the lower leg  80   b . A contact pin  82  is mounted in a recess  84  of the upper leg  80   a  and engages a contact surface  51   c ,  FIGS. 8 and 9 , of the charging member  51 . A first spring  86  is disposed in the recess  84 , biasing the contact pin  82  away from the upper leg  80   a . A second spring  88  is mounted between the lower leg  80   b  and the lower receiver body  19 . The first and the second springs  86 ,  88  have respective spring rates such that the bolt catch  80  is biased to an unlocked position U,  FIG. 9 .  
         [0070]     The bolt  34  and the bolt carrier  36  may be manually moved from the closed position C to the open position O,  FIG. 8 , by moving the charging member  51  in a rearward direction. When the charging member  51  is moved in the rearward direction, the contact pin  82  encounters a contoured portion  51   d  of the charging member  51 . The position of the contoured portion  51   d  relative to the bolt  34  and the profile of the contoured portion  51   d  result in the bolt catch  80  being moved by the charging member  51  to a locked position L,  FIG. 10 , when the bolt  34  is moved to the open position  0 .  
         [0071]     As mentioned above in reference to  FIG. 2C , the bolt  34  and bolt carrier  36  are biased in a forward direction toward the closed position C by the action spring  41   a . Accordingly, when the charging member  51  is moved in the forward direction, the bolt  34  is urged in the forward direction against a locking leg  80   c  by the action spring  41   a . In this manner, the locking leg  80   c  engages a face  34   a  of the bolt  34 , thus holding the bolt  34  and the bolt carrier  36  in the open position  0 . By manually pressing the upper leg  80   a , bolt catch  80  is moved to the unlocked position U, disengaging the locking leg  80   c  from the face  34   a  of the bolt  34  thereby allowing the bolt  34  and bolt carrier  36  to return to the closed position C under the influence of the action spring  41   a.    
         [0072]     Implementation of an embodiment of the bolt catch  80  disclosed herein simplifies the operation of locking the bolt of a firearm in the open position. Many conventional bolt catches, such as that used on M-16 type firearms, require manual manipulation of the bolt catch to lock the bolt in the open position. In situations such as military combat, it is advantageous and desirable to preclude the need to manually manipulate the bolt catch when locking the bolt in the open position. Embodiments of the bolt catch  80  disclosed herein allow the bolt to be locked in the open position without requiring manual manipulation of the bolt catch  80 . The bolt catch  80  described herein, can also be moved automatically from an unlocked position U to a locked position L, by action of a magazine follower from an empty magazine upon a protruding tang (not shown) on the bolt catch  80 . This facilitates the rapid reloading of the weapon when used with ammunition magazines.  
         [0073]     As mentioned above in reference to  FIG. 2E , moving the bolt  34  and the bolt carrier  36  between the open position  0  and the closed position C includes rotating the bolt  34  for unlocking and locking, respectively, the lugs of the bolt  34  from corresponding lugs of the barrel extension.  FIGS. 11-13  show an embodiment of a mechanism for rotating lugs  34   b  of the bolt  34  between the unlocked position U′ and the locked position L′. A cam pin  90  is attached to the bolt  34 . The cam pin  90  is positioned in a cam pin hole  34   c  of the bolt  34 ,  FIG. 13 . The firing pin  32  extends through a firing pin hole  34   d  of the bolt  34  and a firing pin hole  90   a  of the cam pin  90 . The cam pin  90  is captured in a cam slot  92  of the bolt carrier  36 ,  FIGS. 11 and 12 . When the bolt  34  is rotated such that the lugs  34   b ,  FIG. 11 , of the bolt  34  are unlocked from the lugs of the barrel extension, the cam pin  90  is positioned in a first region  92   a  of the cam slot  92 . When the lugs  34   b  are unlocked from the lugs of the barrel extension, a retaining arm  94  is engaged with the cam pin  90  for retaining the cam pin  90  in the first region  92   a  of the cam slot  92 . When the bolt  34  is moved toward the closed position and the bolt  34  engages the barrel extension, a ramp  94   a  of the retaining member  94 ,  FIG. 12 , engages a stationary ramp, thereby pivoting the retaining member  94  for allowing the cam pin  90  to move into a second region  92   b  of the cam slot  92 . A feed tray  96  is a suitable stationary component to which the stationary ramp may be attached. When the cam pin  90  is in the second region  92   b  of the cam slot  92 , the lugs  34   b  of the bolt  34  are in the locked position relative to the lugs of the barrel extension.  
         [0074]     Another embodiment of a mechanism for rotating the lugs  34   b  of the bolt  34  between the unlocked position and the locked position is depicted in  FIGS. 14-17 . In this embodiment, the cam pin  90  extends through the cam pin slot  92  and into the bolt carrier lug channel  42   b  of the upper receiver body  42 . In this manner, the cam pin  90  is constrained to follow a path defined by the bolt carrier lug channel  42   b . When the bolt  34  is in the unlocked position U′,  FIGS. 14 and 15 , the cam pin  90  is positioned in the first region  92   a  of the cam slot  92  and is free to travel in the forward and rearward directions along the length of the bolt carrier lug channel  42   b . When the face  34   a  of the bolt  34  contacts the barrel extension, the bolt carrier  36  continues its forward movement. The continued forward movement of the bolt carrier  36  results in the cam pin  90  rotating in the cam slot  92  to the second region of the cam pin slot  92   b , locking the lugs  34   b  of the bolt  34  relative to the lugs of the barrel extension. The bolt  34  is now in the locked position L′. A relief  42   c  is formed adjacent to the bolt carrier lug channel  42   b  for receiving the cam pin  90  when the bolt  34  is in the locked position L′. The bolt carrier lug  36   a  has a sufficient length such that it cannot rotate into the relief  42   c . A bolt carrier assembly comprises the bolt  34  and the bolt carrier  36 .  
         [0075]     Referring to  FIGS. 18-25 , an ammunition belt feeding assembly  100  is mounted on the upper receiver body  42  of the upper receiver assembly  12 . The ammunition belt feeding assembly  100  and the upper receiver assembly  12  comprise a belt feed receiver system. The ammunition belt feeding assembly  100  includes a top cover  102  mounted adjacent to the feed tray  96 . The top cover  102  and the feed tray  96  are pivotally attached to the upper receiver body  42  through a plurality of bosses  104 . A latch mechanism releasably engages a mounting bracket  106 ,  FIG. 20 , that is attached to the upper receiver body  42 . The feed tray  96  includes a belt channel  96   a  and a link ejection channel  96   b . A feed pin  108 ,  FIG. 20 , is attached to the bolt carrier  36  and extends through a feed pin channel  110  in the upper receiver body  42 . The feed pin  108  moves in unison with the bolt carrier  36  along the feed pin channel  110 .  
         [0076]     The ammunition belt feeding assembly  100  includes a two-stage cam-lever type ammunition belt feeding mechanism  112 ,  FIGS. 21A-21B , attached to the top cover  102 . It is contemplated that other types of cam-lever type ammunition belt feeding mechanisms, such as a single-stage cam-lever type, may be implemented with the upper receiver assembly  12  disclosed herein. It is beneficial for a cam-lever type ammunition belt feeding mechanism to be configured to limit adverse affects associated with acceleration and deceleration of the ammunition belt  114 .  
         [0077]     Referring to  FIGS. 21-25 , a cam lever  113  is pivotally attached to the top cover  102  at a pivot pin  116 . The cam lever  113  includes a cam lever slot  118  having a dwell region  118   a  and a feed region  118   b . The feed pin  108  is received in the cam lever slot  118 . The cam lever  118  is engaged with a feed link  120  for pivoting the feed link  120  about a pivot pin  122 . A first slide member  124  and a second slide member  126  are attached to the feed link  120  at respective feed link pins  124   a ,  126   a . Primary feed pawls  128  are pivotally attached to the first slide member  124  and a secondary feed pawl  130  is pivotally attached to the second slide member  126 . The first slide member  124  and the second slide member  126  include respective guide slots  124   b ,  126   b . A guide pin  132  is attached to the top cover  102  and engages the first and the second slide members  124 ,  126  at the respective guide slots  124   b ,  126   b.    
         [0078]     Still referring to  FIGS. 21-25 , the ammunition belt feeding mechanism  112  operates in two distinct phases and feeds an ammunition belt  114  through the belt channel  96   a  towards the link ejection channel  96   b . When the bolt and bolt carrier begins their forward travel toward the closed position, the feed pin  108  moves in a dwell region  118   a  of the cam lever slot  118  from a first dwell position D 1  to a second dwell position D 2 ,  FIG. 21A . The operation and travel of the bolt and carrier are discussed above. The feed pin  108  is in the dwell region  118   a  of the cam lever slot  118  during a first portion of the forward travel of the bolt and the bolt carrier. While the feed pin  108  is in the dwell region  118   a  of the cam lever slot  118 , the first and the second slide members  125 ,  126  are stationary,  FIGS. 25A and 25B . Thus, the primary and the secondary feed pawls  128 ,  130  remain stationary while the feed pin  108  is in the dwell region  118   a  of the cam lever slot  118 . As depicted in  FIGS. 25A and 25B , a first round  114   a  at a chambering position C 1  is chambered while the feed pin  108  is in the dwell region  118   a  of the cam lever slot  118 . The first round  114   a  is now in a chambered position C 2 , ready for being fired.  
         [0079]     During the second portion of the forward travel of the bolt and the bolt carrier, the feed pin  108  reaches the feed region  118   b  of the cam lever slot  118  and travels from the second dwell position D 2  to a feed position F,  FIG. 21B . As a result of the feed region  118   b  being skewed with respect to the dwell region  118   a , the cam lever  113  pivots from a static position S′,  FIG. 21A , to a displaced position D′,  FIG. 21B , as the feed pin  108  travels from the second dwell position D 2  to the feed position F. The pivoting action of the cam lever  113  pivots the feed link  120 . Accordingly, because the first and the second slide members  124 ,  126  are pinned to the feed link  120  on opposing sides of the pivot pin  122 , the primary feed pawls  128  move towards the chambering position C 1  and the secondary feed pawl  130  moves away from the chambering position C 1 ,  FIGS. 25C and 25D .  
         [0080]     During movement towards the chambering position C 1 , the primary feed pawls  128  advance the second round  114   b  towards the chambering position C 1  and into engagement with a cartridge follower  134 . The cartridge follower  134 ,  FIG. 25D , exerts a downward force on the cartridge of the second round  114   b , biasing the second round  114   b  towards the chambered position C 2 . During movement away from the chambering position C 1 , the secondary feed pawl  130  ratchets over the cartridge of the second round  114   b ,  FIG. 25C . In this manner, when the feed pin  108  reached the feed position F, the second round  114   b  is advanced towards the chambering position C 1  and all of the feed pawls  128 ,  130  are positioned between the second round  114   b  and a third round  114   c ,  FIG. 25D .  
         [0081]     The primary and the secondary feed pawls  128 ,  130  may be biased to an engagement position E,  FIG. 25D , by respective springs, by gravity, or any other suitable means for being automatically returned to the engagement position E after being ratcheted over a cartridge. The travel of the feed pin  108  from the second dwell position D 2  to the feed position F results in the second round  114   b  being advanced approximately a first half of a pitch P of the ammunition belt  114 . The bolt attains its closed position when the feed pin  108  reaches the feed position F.  
         [0082]     After the first round  114   a  is fired, the bolt and the bolt carrier travel rearward towards the open position. The operation and travel of the bolt is discussed above. Accordingly, the feed pin  108  travels from the feed position F towards the second dwell position D 2 . As the feed pin  108  travels from the feed position F toward the second dwell position D 2 , the feed cam-lever  113  pivots from the displaced position D′ to the static position S′. As the feed pin  108  travels from the displaced position D′ to the static position S′, the primary feed pawls  128  move away from the chambering position C 1  and the secondary feed pawl  130  moves towards the chambering position C 1 ,  FIGS. 25D and 25E .  
         [0083]     During movement towards the chambering position C 1 , the secondary feed pawl  130  advance the second round  114   b  to the chambering position C 1 . As the secondary feed pawl  130  advances the second round  114   b  towards the chambering position C 1 , the cartridge follower  134  exerts additional force on the cartridge of the second round  114   b , further biasing the second round  114   b  towards the chambered position C 2 . During movement away from the chambering position C 1 , the primary feed pawls  128  ratchet over the cartridge of the third round  114   c . The second round  114   b  is now positioned at the chambering position C 1 ,  FIG. 25E . The secondary feed pawl  130  is now positioned between the second round  114   b  and the third round  114   c . The primary feed pawls  128  are now positioned between the third round  114   c  and a fourth round  114   d . The travel of the feed pin  108  from the feed position F to the second dwell position D 2  results in the second round  114   b  being advanced a second half of the pitch P of the ammunition belt  114 . The feed pawls  128 ,  130  do not move as the feed pin  108  travels from the second dwell position D 2  back to the first dwell position D 1 .  
         [0084]     Referring to  FIGS. 26-28 , an embodiment of a sprocket type ammunition belt feeding mechanism  212  includes a feed sprocket  215  and a drive shaft assembly  216  coupled to the feed sprocket  215 . As depicted in  FIG. 26 , a mounting shaft  213  extends through the feed sprocket  215  and drive shaft assembly  216 , permitting the feed sprocket  215  and the drive shaft assembly  216  to rotate relative to a top cover  202  of an ammunition belt feeding assembly. The mounting shaft  213  is attached to the top cover  202  via a first and a second mounting bracket  217   a ,  217   b . At least one of the mounting brackets  217   a ,  217   b  is removable from the top cover  202  for permitting the ammunition belt feeding mechanism  212  to be detached from the top cover  202 .  
         [0085]     In an alternated embodiment (not shown), the feed sprocket  215  and the drive shaft assembly  216  are mounted on a common axle shaft. The common axle shaft extends through the feed assembly and top cover ends. The axle shaft is secured by a cross-pin through the cover and radius of the axle shaft on one end of the cover.  
         [0086]     The drive shaft assembly  216 ,  FIGS. 26 and 27 , includes a drive shaft  218  and a drive sleeve  220  mounted in a counter-bored end  218   a  of the drive shaft  218 . The feed sprocket  215  includes a drive hub  215   a  that is fixedly attached to the feed sprocket  215  such that the feed sprocket  215  is precluded from rotating relative to the drive hub  215   a . The drive sleeve  220  includes a plurality of ribs  220   a  thereon that mate with corresponding grooves  218   b  of the drive shaft  218  such that the drive sleeve  220  is precluded from rotating relative to the drive shaft  218 . A spring  222 ,  FIG. 27 , is mounted between the drive sleeve  220  and the drive shaft  218  for biasing the drive sleeve  220  into engagement with the drive hub  215   a  of the feed sprocket  215 ,  FIG. 26 . The drive sleeve  220  and the drive hub  215   a  have mating tapered teeth. Accordingly, the drive shaft  218  can rotate relative to the feed sprocket  215  in only one direction.  
         [0087]     An operational cycle of the ammunition belt feeding mechanism  212  begins with a first round  214   a  being stripped from the ammunition belt  214  at the chambering position C 1  by the bolt and chambered into the firing chamber,  FIG. 28A . The first round  214   a  is now at the chambered position C 2 . After the first round  214   a  is fired, the bolt and bolt carrier travel from the closed position toward the open position. The drive shaft  218  includes a spiral drive slot  218   c  that receives the feed pin of the bolt carrier (discussed above). The profile of the drive slot  218   c  may be configured for minimize adverse affects associated with acceleration and deceleration of the ammunition belt  214 .  
         [0088]     As the bolt carrier travels towards the open position, the feed pin travels in the drive slot  218   c  of the drive shaft  218 , rotating the drive shaft  218  and the feed sprocket  215  from the static position S″,  FIG. 28A , to the rotated position R″,  FIG. 28B . The profile of the drive slot  218   c  is configured for rotating the drive shaft  218  through an angular displacement corresponding to the pitch P of the ammunition belt  214 . Accordingly, a second round  214   b  is advanced to the chambering position C 1  during rotation of the drive shaft from the static position S″ to the rotated position R″. The cartridge of the first round  214   a  is withdrawn from the firing chamber and is ejected from the firearm as the bolt carrier travels from the closed position towards the open position.  
         [0089]     An action spring (discussed above) arrests the travel of the bolt carrier toward the open position and urges the bolt carrier towards the closed position. As the bolt carrier travels from the open position toward the closed position, the drive shaft  218  rotates from the rotated position R″ back to the static position S″,  FIG. 28C . An anti-reverse member  224  is engaged with the feed sprocket  215 . The anti-reverse member  224  provides a retention force on the feed sprocket  215 , holding the feed sprocket  215  stationary while the drive shaft  218  rotates back to the static position S″.  
         [0090]     In the preceding detailed description, reference has been made to the accompanying drawings which form a part hereof, and in which are depicted by way of illustration specific embodiments in which the invention may be practiced. These embodiments, and certain variants thereof, have been described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other suitable embodiments may be utilized and that logical, mechanical, chemical and electrical changes may be made without departing from the spirit or scope of the invention. For example, functional blocks depicted in the figures could be further combined or divided in any manner without departing from the spirit or scope of the invention. To avoid unnecessary detail, the description omits certain information known to those skilled in the art. The preceding detailed description is, therefore, not intended to be limited to the specific forms set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the appended claims.