Patent Publication Number: US-10323891-B1

Title: Extremely short buffer system and bolt carrier design for firearms

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to firearms. More specifically, the present disclosure relates to an extremely short buffer system and bolt carrier design for firearms. 
     BACKGROUND 
     A bolt carrier group (BCG)  100  of a traditional AR15-type firearm, as shown in  FIG. 13 , usually consists of a bolt carrier  103 , a bolt assembly  101 , a firing pin  105 , a cam pin  102 , and a firing pin retaining pin  104 . For a direct impingement type of gas system to operate, hot gas directed from a gas tube (not shown) of a firearm would flow into the bolt carrier  103  through a gas channel  106  on the bolt carrier  103  and into the chamber (not shown) inside the bolt carrier  103  and push the bolt assembly  101  to rotate under a force generated by the cam pin  102 . The hot gas typically has a mild pressure compared to the pressure in the gun barrel chamber; however, it is still considered as high pressure compared to ambient air pressure. A length  107  is the dimension between a front face of the bolt carrier  103  and a full auto actuation surface  108  on a rear face of the bolt carrier  103 . 
     Referring to  FIG. 14 - FIG. 16 , a lower assembly of a traditional AR15 firearm consists of a lower receiver  201 , a buffer  202 , a main spring  203 , an end plate  204 , a castle nut  205 , a buffer tube  206 , and a stock  207 . The buffer tube  206  houses the buffer  202  and main spring  203  therein, while the castle nut  205  fastens the end plate  204  and the buffer tube  206  to the lower receiver  201 . Once the firearm fires one cartridge, the BCG would retract back toward the back of the firearm and compress the main spring  203 . A tail portion of the bolt carrier  103  would sink into or otherwise be received in the buffer tube  206 . The stock  207  is attached to the buffer tube  206  though its own latching mechanism (not shown), such that the stock  207  can slide forward and backward as well as fasten to the buffer tube  206  at various points on the buffer tube  206 , so that adjustment can be made to satisfy a specific length of arms of a user to allow the user to operate the firearm comfortably. 
     Current designs of AR15 firearms demand a buffer tube in various lengths greater than 7 to 8 inches in order for the direct impingement type of gas system to function properly. As such, the buffer tube  206 , and the stock that  207  that is attached to it, cannot be folded to one side of the firearm to shorten the weapon system for carrying by a user in various compact situations, some of which would demand the weapon to be packed as short as possible. 
     Presently there are some folding stock adaptor designs that can fold the buffer tube and stock of a firearm with a direct impingement type of gas system. However, with such designs, when the buffer tube/stock is folded it is not safe to fire the firearm, thereby rendering the firearm useless after firing a first round of cartridge. In order for such designs to function, the buffer tube/stock needs to be unfolded to be in a combat configuration for the system to operate properly. 
     There exist other designs for shortening the buffer tube and the bolt carrier. However, those designs tend to render disassembly of the firearm difficult. 
     SUMMARY 
     The present disclosure proposes a novel design of an extremely short buffer system and bolt carrier design for firearms that addresses aforementioned issued with existing designs. 
     In one aspect, a device implementable in an AR-15 styled firearm may include a buffer tube and spring system and a bolt carrier assembly. The buffer tube and spring system may be configured to be mounted on a rear end of a lower receiver of the firearm. The buffer tube and spring system may include a pusher and a buffer tube. The buffer tube may include a cavity configured to receive the buffer therein. The bolt carrier assembly may include a bolt carrier head and a bolt carrier tail. A rear end of the bolt carrier tail may be connected to the pusher. A front end of the bolt carrier head may include a cavity configured to house a bolt assembly of the firearm therein. A front end of the bolt carrier tail may be elastically coupled to a rear end of the bolt carrier head such that, during operation of the firearm: (1) the bolt carrier head travels linearly at most by a first dimension, (2) the bolt carrier tail travels linearly at most by a second dimension smaller than the first dimension, and (3) the pusher travels linearly at most by a third dimension smaller than the second dimension. 
     These and other objectives of the present disclosure will be appreciated by those of ordinary skill in the art after reading the following detailed description of the preferred embodiments that are illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of the present disclosure. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. It is appreciable that the drawings are not necessarily in scale as some components may be shown to be out of proportion than the size in actual implementation in order to clearly illustrate the concept of the present disclosure. 
         FIG. 1  is a diagram of a bolt carrier assembly in accordance with an embodiment of the present disclosure. 
         FIG. 2  is a diagram of a first view of a bolt carrier head in accordance with an embodiment of the present disclosure. 
         FIG. 3  is a diagram of a second view of the bolt carrier head of  FIG. 2 . 
         FIG. 4  is a diagram of a pusher/bolt carrier tail in accordance with an embodiment of the present disclosure. 
         FIG. 5  is a diagram of a comparison between a conventional bolt carrier and a bolt carrier assembly in accordance with an embodiment of the present disclosure. 
         FIG. 6  is a diagram of a lower receiver with a shortened buffer tube and spring system in accordance with an embodiment of the present disclosure. 
         FIG. 7  is a diagram of an exploded view of a buffer tube and spring system mounted to a lower receiver in accordance with an embodiment of the present disclosure. 
         FIG. 8  is a diagram of an exploded view of a conventional spring assembly of a pistol. 
         FIG. 9  is a diagram showing a relationship between a bolt carrier tail and a pusher in accordance with an embodiment of the present disclosure. 
         FIG. 10  is a diagram of a BCG assembly with a main spring fully extended in accordance with an embodiment of the present disclosure. 
         FIG. 11  is a diagram of a BCG assembly with a main spring fully compressed in accordance with an embodiment of the present disclosure. 
         FIG. 12  is a diagram of a buffer tube design in accordance with an embodiment of the present disclosure. 
         FIG. 13  is a diagram of a conventional BCG of an AR15 firearm. 
         FIG. 14  is a diagram of a conventional BCG in a battery position in relation to an AR15 lower assembly. 
         FIG. 15  is a diagram of an exploded view of a conventional AR15 bolt carrier with a lower assembly. 
         FIG. 16  is a diagram of a conventional BCG in a retracted position when traveling backwards to a rear-most position in relation to an AR15 lower assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     The position terms used in the present disclosure, such as “front”, “forward”, “rear”, “back”, “top”, “bottom”, “left”, “right”, “head”, “tail” or the like assume a firearm in the normal firing position, with the firearm being in a position in which the longitudinal axis of the barrel of the firearm runs generally horizontally and the direction of firing points “forward” away from the operator of the firearm. The same convention applies for the direction statements used herein. 
     Overview 
       FIG. 1  illustrates a bolt carrier assembly  300  in accordance with an embodiment of the present disclosure. Referring to  FIG. 1 , bolt carrier assembly  300  includes a bolt carrier head  301 , two bolt carrier spring guide rods  302 , two bolt carrier/main springs  303 , and a buffer tail or bolt carrier tail  304 . On one hand, bolt carrier head  301  retains major design features of bolt carrier  103 , especially the front portion thereof. For instance, a front end of bolt carrier head  301  includes a cylindrical cavity configured to house bolt assembly  101  therein. On the other hand, compared to bolt carrier  103 , bolt carrier head  301  is shortened almost all the way to the housing of firing pin  105  is located on bolt carrier  103 , where the pointer of firing pin  105  is pointing in  FIG. 13 . 
       FIG. 2  and  FIG. 3  each illustrates a respective view of bolt carrier head  301 . Referring to  FIG. 2 , bolt carrier head  301  includes two clearance grooves  401  at the front-most portion of bolt carrier head  301 . Bolt carrier head  301  also includes two clearance grooves  403  on two opposite sides of a rear portion of bolt carrier head  301 . Each of the two clearance grooves  403  is configured to receive or otherwise accommodate one distal end of a respective one of the two bolt carrier/main springs  303 . Bolt carrier head  301  further includes two guide rod clearance holes  402  on two opposite sides of bolt carrier head  301  and between clearance grooves  401  and clearance grooves  403 . Each of the two bolt carrier spring guide rods  302  can slide freely within a respective one of the clearance grooves  401 , clearance grooves  403  and guide rod clearance holes  402 . The two bolt carrier spring guide rods  302  provide guidance for bolt carrier/main springs  303  to compress and extend during operation (e.g., during the cycling of bolt carrier head  301 ). 
     It is noteworthy that a magazine relief cut area  404  on bolt carrier head  301  is the thinnest area on bolt carrier head  301 . With respect to the wall thickness at each of the clearance grooves  401 , it is imperative to design the wall thickness  405  to be no less than that of magazine relief cut area  404 . In some implementations, wall thickness  405  is thicker or greater than that of magazine relief cut area  404  by at least 0.014 inch. 
     Referring to  FIG. 3 , each of the two clearance grooves  403  has a surface that is on the same plane as that of a bottom plane of a firing pin housing pocket  406  on bolt carrier head  301 , which is the surface of a pressure chamber within bolt carrier head  301  subject to the internal pressure from explosion of a round of ammunition. Accordingly, new design features on bolt carrier head  301  would not undermine the strength and structural integrity of the wall of the pressure chamber in the direction perpendicular to the surface. 
       FIG. 4  illustrates a buffer tail or bolt carrier tail  304  in accordance with an embodiment of the present disclosure. Bolt carrier tail  304  includes two cavity grooves  407 . Each of the two cavity grooves  407  is configured to receive or otherwise accommodate the other distal end of a respective one of the two bolt carrier/main springs  303 . Bolt carrier tail  304  also includes recess areas  408  configured to interact or otherwise come in contact with the rear end of bolt carrier head  301 . Bolt carrier tail  304  is configured with four relief cuts  409  that make rooms for the structure of a newly designed buffer tube to be described below. Bolt carrier tail  304  further includes features such as a front lip surface  410 , a clearance hole  411 , a groove  412 , two holes  413  which may be through holes or threaded holes, a hook  414 , and a gap  415 . Clearance hole  411  is configured to allow operation of a buffer spring guide rod. Groove  412  is similar to that on bolt carrier  103 . The two holes  413  are configured to allow bolt carrier spring guide rods  302  to be connected to bolt carrier tail  304 . That is, each of bolt carrier spring guide rods  302  may be threaded on one end to engage with a respective one of the holes  413 . Alternatively, an end of the bolt carrier spring guide rods  302  may be bent after it is inserted to pass through the hole  403  as a through hole, such that front tip surface  410  of bolt carrier tail  304  may be in a position identical to a full-auto actuation surface on bolt carrier  103 , while the other end of bolt carrier spring guide rods  302  may be bent so that it can be retained inside clearance groove  401 . 
       FIG. 5  illustrates a comparison between a conventional bolt carrier and a bolt carrier assembly in accordance with an embodiment of the present disclosure. Part (A) of  FIG. 5  shows a conventional bolt carrier, and part (B) of  FIG. 5  shows bolt carrier assembly  300 . Referring to  FIG. 5 , bolt carrier assembly  300  is assembled in a way that front tip surface  410  would be at the same position as that of tail of bolt carrier  103  such that full-auto capability of bolt carrier  103  is retained in bolt carrier assembly  300 . 
       FIG. 6  illustrates lower receiver  201  with a shortened buffer tube and spring system  500  in accordance with an embodiment of the present disclosure. The buffer tube and spring system  500  is designed to provide bolt carrier assembly  300  an extra compression length (in addition to the main spring compression length) that is needed for bolt carrier head  301  to move to its furthest rearward position. Referring to  FIG. 6 , a buffer tube  501  of buffer tube and spring system  500  may be fastened to lower receiver  201  with end plate  204  and castle nut  205 . In some implementations, buffer tube  501  may be fastened to lower receiver  201  with a folding stock adaptor (not shown) and a different kind of castle not (not shown). 
       FIG. 7  illustrates an exploded view of buffer tube and spring system  500  mounted to lower receiver in accordance with an embodiment of the present disclosure. Buffer tube and spring system  500  is designed to provide bolt carrier assembly  300  with necessary retraction space when bolt carrier assembly  300  is compressed during operation. Buffer tube and spring system  500  includes an inner sliding guide rod  502 , a guide rod spring  503 , a pusher  504 , an outer spring  505 , an outer spring/inner spring coupler  506 , an inner spring  507 , a fixed inner guide rod  508 , and a retainer clamp spring  509  configured to be coupled to inner sliding guide rod  502 . Hook  414  of bolt carrier tail  304  may connect with pusher  504 . The two inner walls of gap  415  may be in contact with pusher  504  during different stages of operation of bolt carrier assembly  300  as it moves inside an upper receiver (not shown) of the firearm. 
       FIG. 8  illustrates an exploded view of a conventional spring assembly  600  of a pistol. Buffer tube and spring system  500  may be, in some way, similar to spring assembly  600 , which includes an inner guide rod  601 , an outer spring  602 , an inner spring/outer spring coupler  603 , and an inner spring  604 . One main difference between buffer tube and spring system  500  and spring assembly  600  may be that, buffer tube and spring system  500 , the inner guide rod  601  is replaced with a three-part assembly including inner sliding guide rod  502 , guide rod spring  503  and fixed inner guide rod  508 . This design is to provide space for retraction of bolt carrier assembly  300 . In comparison, the design of spring assembly  600  would interfere with bolt carrier assembly  300 . 
       FIG. 9  illustrates a relationship between bolt carrier tail  304  and a pusher  504  in accordance with an embodiment of the present disclosure. Pusher  504  is designed to engage with outer spring  505 , the head of inner spring guide rod  502 , and bolt carrier tail  304 . When traveling together with bolt carrier tail  304 , pusher  504  may constantly come in contact with outer spring  505 . When the upper receiver is open from the lower receiver  201 , pusher  504  may be retained by inner spring guide rod  502  under the pressure from outer spring  505 . When the upper receiver is closed to lower receiver  201 , pusher  504  may come in contact with bolt carrier tail  304  in two different positions as shown in  FIG. 9 . Part (A) of  FIG. 9  shows the rear side of bolt carrier tail  304  being in contact with pusher  504 , and this is the usual relationship between bolt carrier tail  304  and pusher  504 . Part (B) of  FIG. 9  shows bolt carrier tail  304  moved forward to its most frontward position under its own forward moving momentum when the bolt carrier head  301  impact to the end of barrel extension (not shown). In BCG  100 , bolt carrier  103  tends to bounce back a little when impact onto the end of barrel extension (not shown), causing a “bolt bounce” problem which may affect the full-auto function of the firearm. This problem is alleviated with the introduction of buffer  202 , which can include several heavy metallic discs or cylinders (not shown) that add weight/impact to bolt carrier and hit the buffer  202 , hence transfer the force to the bolt carrier  103 , under its momentum when bolt carrier  103  is stopped by the barrel extension (not shown), before bolt carrier  103  bounces back from the barrel extension. 
     In buffer tube and spring system  500 , however, there is no room for buffer  202 . Dur to the connection of bolt carrier/main spring  303  and the comprehensive buffer spring behind it, when bolt carrier head  301  impacts on the barrel extension, bolt carrier tail  304  may continue traveling forward under its own momentum. The momentum and force would transfer to bolt carrier head  301  through bolt carrier/main spring  303 , which would function as a buffer. Hook  414  may limit the travel distance of bolt carrier tail  304  (e.g., limited by pusher  504 ) to avoid resonance with bolt carrier head  301 . 
       FIG. 10  illustrates a BCG assembly with a main spring fully extended with respect to lower receiver  102  in accordance with an embodiment of the present disclosure.  FIG. 11  illustrates a BCG assembly with a main spring fully compressed with respect to lower receiver  102  in accordance with an embodiment of the present disclosure. With this design, an extremely short buffer tube together with the new BCG design would allow normal operation (including full-auto capability) and disassembly of the firearm. 
       FIG. 12  illustrates a design of buffer tube  501  in accordance with an embodiment of the present disclosure. Part (A) of  FIG. 12  shows that buffer tube  501  includes four ribs  701  that support a rear flange, which leaves four clearance holes  704 . Buffer tube  501  also includes a buffer tube main shaft  702  that allows bolt carrier tail  304  to retract and be received therein. Buffer tube  501  further includes a clearance groove  703  that gives way to a buffer retaining pin (not shown) on lower receiver  102 . Clearance groove  703  also serves as an indexing key for buffer tube  501  to align correctly with lower receiver  102  before buffer tube  501  is tightened to position by castle nut  205 . Buffer tube  501  may be designed to be sealed on one end as shown in part (B) of  FIG. 12 , although the length of buffer tube  501  would be slightly longer to accommodate pusher  504  and bolt carrier tail  304  therein without contamination of foreign objects. 
     In view of the above, it is believed that those with ordinary skill in the art would appreciate a number of design features presented herein. For instance, the clearance grooves for bolt carrier springs are located behind the pressure chamber inside the bolt carrier head for safety reasons. Moreover, the spring guide rod clearance grooves on the bolt carrier do not weaken the pressure chamber more than that of the magazine clearance groove. This is an important feature as there are other prior designs that would allow the spring groove to pass through the pressure chamber, which would weaken the structure more than the proposed design of the present disclosure. Additionally, the design of bolt carrier tail retains the full-auto capability while functioning as a buffer. The pusher provides a vertical support to prevent the buffer tube inner spring guide rod from bending during assembly and disassembly of the firearm. The pusher also provides a limit on axial movement for the bolt carrier tail to prevent resonance with the bolt carrier head. Furthermore, the design of ribs on the buffer tube would shorten the operation length of the system. The sliding inner spring guide rod and its fixed base are designed to make rooms for the firing pin and its housing on the bolt carrier head during retraction and compression of the bolt carrier assembly. Moreover, the design of the bolt carrier tail allows a portion of the main spring to overlap with the buffer spring system to further shorten the overall length of the bolt carrier assembly proposed herein. 
     Highlights of Select Features 
     In one aspect, a device implementable in an AR-15 styled firearm may include a buffer tube and spring system and a bolt carrier assembly. The buffer tube and spring system may be configured to be mounted on a rear end of a lower receiver of the firearm. The buffer tube and spring system may include a pusher and a buffer tube. The buffer tube may include a cavity configured to receive the bolt tail and the pusher therein. The bolt carrier assembly may include a bolt carrier head and a bolt carrier tail. A rear end of the bolt carrier tail may be connected to the pusher. A front end of the bolt carrier head may include a cavity configured to house a bolt assembly of the firearm therein. A front end of the bolt carrier tail may be elastically coupled to a rear end of the bolt carrier head such that, during operation of the firearm: (1) the bolt carrier head travels linearly at most by a first dimension, (2) the bolt carrier tail travels linearly at most by a second dimension smaller than the first dimension, and (3) the pusher travels linearly at most by a third dimension smaller than the second dimension. 
     In some implementations, the bolt carrier assembly may further include a bolt carrier spring guide rod and a bolt carrier spring around the bolt carrier spring guide rod. At least one side of the bolt carrier head may include a first clearance groove, a second clearance groove and a guide rod clearance hole between the first clearance groove and the second clearance groove such that a first distal end of the bolt carrier spring guide rod traverses through the guide rod clearance hole to be slidingly received in the first clearance groove and the second clearance groove. At least one side of the bolt carrier tail may include a cavity groove and a hole such that a second distal end of the bolt carrier spring guide rod opposite the first distal end thereof is connected to the bolt carrier tail through threads or by bending a tip of the second distal end of the bolt carrier spring guide rod. The bolt carrier spring may be slidingly received in the second clearance groove of the bolt carrier head and the cavity groove of the bolt carrier tail. 
     In some implementations, a surface of the guide rod clearance hole facing the bolt carrier spring may be on a same plane as a surface of a pressure chamber within the bolt carrier head that is subject to an internal pressure from explosion of a round of ammunition during operation of the firearm. 
     In some implementations, the buffer tube and spring system may further include: an outer spring/inner spring coupler, an outer spring around the outer spring/inner spring coupler, a fixed inner guide rod coupled to the outer spring/inner spring coupler, an inner spring around the fixed inner guide rod, an inner sliding guide rod slidingly received in a hollow of the fixed inner guide rod, a guide rod spring around the inner sliding guide rod, and a retainer clamp spring configured to be couple a distal end of the inner sliding guide rod to the buffer tube. 
     In some implementations, the pusher may provide vertical support to the inner spring guide rod such that bending of the inner spring guide rod is prevented during assembly and disassembly of the firearm. 
     In some implementations, at least a portion of the bolt carrier spring may overlap with the buffer tube and spring system along a longitudinal axis of the lower receiver. 
     In some implementations, the pusher may limit a linear movement of the bolt carrier tail along a longitudinal axis of the lower receiver such that a resonance between the bolt carrier tail and the bolt carrier head during operation of the firearm is dampened. 
     In some implementations, the buffer tube may include a plurality of ribs. 
     In some implementations, the bolt carrier head may include a magazine relief cut area. A wall thickness between the first clearance groove and the cavity at the front end of the bolt carrier head may be greater than a wall thickness between the magazine relief cut area and the cavity at the front end of the bolt carrier head. 
     In some implementations, the wall thickness between the first clearance groove and the cavity at the front end of the bolt carrier head may be greater than the wall thickness between the magazine relief cut area and the cavity at the front end of the bolt carrier head by at least 0.014 inch. 
     In another aspect, an AR-15 styled firearm may include a lower receiver, a bolt assembly, a buffer tube and spring system and a bolt carrier assembly. The buffer tube and spring system may be mounted on a rear end of the lower receiver. The buffer tube and spring system may include a pusher and a buffer tube. The buffer tube may include a cavity configured to receive the bolt carrier tail and the pusher therein. The bolt carrier assembly may include a bolt carrier head and a bolt carrier tail. A rear end of the bolt carrier tail may be connected to the pusher. A front end of the bolt carrier head may include a cavity configured to house the bolt assembly therein. A front end of the bolt carrier tail may be elastically coupled to a rear end of the bolt carrier head such that, during operation of the firearm: (1) the bolt carrier head travels linearly at most by a first dimension, (2) the bolt carrier tail travels linearly at most by a second dimension smaller than the first dimension, and (3) the pusher travels linearly at most by a third dimension smaller than the second dimension. 
     In some implementations, the bolt carrier assembly may further include a bolt carrier spring guide rod and a bolt carrier spring around the bolt carrier spring guide rod. At least one side of the bolt carrier head may include a first clearance groove, a second clearance groove and a guide rod clearance hole between the first clearance groove and the second clearance groove such that a first distal end of the bolt carrier spring guide rod traverses through the guide rod clearance hole to be slidingly received in the first clearance groove and the second clearance groove. At least one side of the bolt carrier tail may include a cavity groove and a hole such that a second distal end of the bolt carrier spring guide rod opposite the first distal end thereof is connected to the bolt carrier tail through threads or by bending a tip of the second distal end of the bolt carrier spring guide rod. The bolt carrier spring may be slidingly received in the second clearance groove of the bolt carrier head and the cavity groove of the bolt carrier tail. 
     In some implementations, a surface of the guide rod clearance hole facing the bolt carrier spring may be on a same plane as a surface of a pressure chamber within the bolt carrier head that is subject to an internal pressure from explosion of a round of ammunition during operation of the firearm. 
     In some implementations, the buffer tube and spring system may further include: an outer spring/inner spring coupler, an outer spring around the outer spring/inner spring coupler, an fixed inner guide rod coupled to the outer spring/inner spring coupler, an inner spring around the fixed inner guide rod, an inner sliding guide rod slidingly received in a hollow of the fixed inner guide rod, a guide rod spring around the inner sliding guide rod, and a retainer clamp spring configured to be couple a distal end of the inner sliding guide rod to the buffer tube. 
     In some implementations, the pusher may provide vertical support to the inner spring guide rod such that bending of the inner spring guide rod is prevented during assembly and disassembly of the firearm. 
     In some implementations, at least a portion of the bolt carrier spring may overlap with the buffer tube and spring system along a longitudinal axis of the lower receiver. 
     In some implementations, the pusher may limit a linear movement of the bolt carrier tail along a longitudinal axis of the lower receiver such that a resonance between the bolt carrier tail and the bolt carrier head during operation of the firearm is dampened. 
     In some implementations, the buffer tube may include a plurality of ribs. 
     In some implementations, the bolt carrier head may include a magazine relief cut area. A wall thickness between the first clearance groove and the cavity at the front end of the bolt carrier head may be greater than a wall thickness between the magazine relief cut area and the cavity at the front end of the bolt carrier head. 
     In some implementations, the wall thickness between the first clearance groove and the cavity at the front end of the bolt carrier head may be greater than the wall thickness between the magazine relief cut area and the cavity at the front end of the bolt carrier head by at least 0.014 inch. 
     In some implementations, the firearm may further include a foldable stock attached to the rear end of the lower receiver. When the foldable stock is folded to one side of the lower receiver, the bolt carrier assembly and the buffer tube and spring system may allow the firearm to operate by firing one or more rounds of ammunition. 
     Additional Notes 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the above-described embodiments or spirit of the present disclosure. Moreover, although examples given in the present disclosure are directed to firearms and usage of the embodiments for firearms, there is no limit on the applications of embodiments of the embodiments disclosed herein. That is, any suitable implementation or application using an embodiment of the present disclosure, or variation thereof, is still within the scope of the present disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of the present disclosure in view of the scope of the following claims and their equivalents.