Patent Publication Number: US-2023143657-A1

Title: Hybrid ambidextrous receiver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. Application Serial No. 16/950,562 (“the ‘562 application”) filed Nov. 17, 2020, which is related to and claims priority benefit from U.S. Provisional Application No. 62/936,555 (“the ‘555 application”), filed on Nov. 17, 2019 and U.S. Provisional Application No. 63/114,253 (“the ‘253 application”), filed on Nov. 16, 2020. The ‘562 application, the ‘555 application and the ‘253 application are each hereby incorporated in its entirety by this reference. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention relates to firearms, particularly receivers for firearms where the receiver is a hybrid design using multiple materials and the receiver is designed with multiple ambidextrous features. 
     BACKGROUND 
     Many modern firearms and firearm accessories (including handguns, rifles, carbines, shotguns, etc.) are designed based on existing modular firearm systems. For example, many firearms and related accessories are designed for compatibility with the AR-15 variant (civilian) or M16/M4 (military) firearm platform. Many of these products follow traditional designs based on industry standards and/or military specification (milspec). However, many of the existing components are not compatible with ambidextrous features, are not optimized for different or multiple materials, and require labor-intensive construction and assembly techniques. U.S. Pat. Nos. 9,297,599 and 9,389,033 describe hybrid receiver designs. Each of these two patents is hereby incorporated in its entirety by this reference. 
     To increase comfort and convenience for a greater number of operators, it may be desirable to design new firearm components or accessories with ambidextrous features. Manufacturing methods utilizing multiple materials to create hybrid parts facilitate the use of specialized materials that more efficiently distribute and dissipate energy while better absorbing vibration and reducing weight for the firearm. Such designs may result in modular firearm components or accessories that increase reliability, reduce perceived recoil, increase safety, and reduce manufacturing/assembly costs. 
     SUMMARY 
     The terms “invention,” “the invention,” “this invention” and “the present invention” used in this patent are intended to refer broadly to all of the subject matter of this patent and the patent claims below. Statements containing these terms should be understood not to limit the subject matter described herein or to limit the meaning or scope of the patent claims below. Embodiments of the invention covered by this patent are defined by the claims below, not this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this patent, any or all drawings and each claim. 
     According to certain embodiments of the present invention, a firearm receiver assembly comprises: a receiver body; a threaded mount at a rear portion of the receiver body; a magazine release assembly comprising a magazine release portion on at least one side of the receiver body; a bolt release assembly comprising a bolt release central portion and a bolt release portion on at least one side of the receiver body; a safety selector assembly comprising a safety portion on at least one side of the receiver body, wherein the bolt release central portion translates vertically within a cavity of the receiver body. 
     According to certain embodiments of the present invention, a lower receiver assembly for an AR-15 style firearm comprises: a receiver body comprising a left side and a right side; a threaded mount at a rear portion of the receiver body; a magazine release assembly comprising a first magazine release portion on the left side of the receiver body and a second magazine release portion on the right side of the receiver body; and a bolt release assembly comprising a bolt release portion on at least one side of the receiver body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a front right perspective view of a firearm according to certain embodiments of the present invention. 
         FIG.  1 B  is a front left perspective view of the firearm of  FIG.  1 A . 
         FIG.  2 A  is a front left perspective view of a receiver assembly of the firearm of  FIG.  1 A . 
         FIG.  2 B  is a rear right perspective view of the receiver assembly of  FIG.  2 A . 
         FIG.  3    is an exploded perspective view of the receiver assembly of  FIG.  2 A . 
         FIG.  4 A  is a front left perspective view of a magazine release assembly and a bolt release assembly of the firearm of  FIG.  1 A . 
         FIG.  4 B  is a rear right perspective view of the magazine release assembly and the bolt release assembly of  FIG.  4 A . 
         FIG.  5 A  is a partial perspective view of the left side of a receiver body of the firearm of  FIG.  1 A . 
         FIG.  5 B  is a partial perspective view of the right side of the receiver body of  FIG.  5 A . 
         FIG.  6 A  is a front left perspective view of a safety selector assembly of the firearm of  FIG.  1 A . 
         FIG.  6 B  is an exploded perspective view of the safety selector assembly of  FIG.  6 A . 
         FIG.  6 C  is a partial perspective view of the right side of the receiver body of the firearm of  FIG.  1 A . 
         FIG.  6 D  is a perspective view of a selector cap of the safety selector assembly of  FIG.  6 A . 
         FIG.  6 E  is a perspective view of a detent clip of the safety selector assembly of  FIG.  6 A . 
         FIGS.  6 F and  6 G  are perspective views of the selector cap and a selector shaft of the safety selector assembly of  FIG.  6 A . 
         FIG.  7 A  is a front right perspective view of a firearm according to certain embodiments of the present invention. 
         FIG.  7 B  is a front left perspective view of the firearm of  FIG.  7 A . 
         FIG.  8 A  is a front left perspective view of a receiver assembly of the firearm of  FIG.  7 A . 
         FIG.  8 B  is a rear right perspective view of the receiver assembly of  FIG.  8 A . 
         FIG.  9    is an exploded perspective view of the receiver assembly of  FIG.  8 A . 
         FIG.  10 A  is a front left perspective view of a receiver body of the receiver assembly of  FIG.  8 A . 
         FIG.  10 B  is a rear right perspective view of the receiver body of  FIG.  10 A . 
         FIG.  11 A  is a front right perspective view of a magazine release assembly and a bolt release assembly of the firearm of  FIG.  7 A . 
         FIG.  11 B  is a rear left perspective view of the magazine release assembly and the bolt release assembly of  FIG.  11 A . 
         FIG.  12 A  is a perspective exploded view of a bolt release assembly of the firearm of  FIG.  7 A . 
         FIG.  12 B  is a perspective exploded view of a magazine release assembly of the firearm of  FIG.  7 A . 
         FIG.  13 A  is a partial perspective view of the left side of a receiver body of the firearm of  FIG.  7 A . 
         FIG.  13 B  is a partial perspective view of the right side of the receiver body of  FIG.  13 A . 
         FIG.  14    is a perspective view of a long rocker of the firearm of  FIG.  7 A . 
         FIGS.  15 A and  15 B  are perspective views of a lifter of the firearm of  FIG.  7 A . 
         FIG.  16    is a rear right lower perspective partial view of a receiver assembly of the firearm of  FIG.  1 A . 
         FIG.  17 A  is a front left perspective view of a threaded mount of the firearm of  FIG.  1 A . 
         FIG.  17 B  is a rear right perspective view of the threaded mount of  FIG.  17 A . 
         FIG.  18    is a rear right perspective view of a threaded mount of the firearm of  FIG.  1 A . 
     
    
    
     DETAILED DESCRIPTION 
     The subject matter of embodiments of the present invention is described here with specificity to meet statutory requirements, but this description is not necessarily intended to limit the scope of the claims. The claimed subject matter may be embodied in other ways, may include different elements or steps, and may be used in conjunction with other existing or future technologies. This description should not be interpreted as implying any particular order or arrangement among or between various steps or elements except when the order of individual steps or arrangement of elements is explicitly described. 
     Although the illustrated embodiments in  FIG.  1 A -18 show components of various semi-automatic or automatic rifles, the features, concepts, and functions described herein are also applicable (with potential necessary alterations for particular applications) to handguns, rifles, carbines, shotguns, or any other type of firearm. Furthermore, the embodiments may be compatible with various calibers including rifle calibers such as, for example, 5.56×45 mm NATO, .223 Remington, 7.62×51 mm NATO, .308 Winchester, 7.62×39 mm, 5.45×39 mm; pistol calibers such as, for example, 9×19 mm, .45 ACP, .40 S&amp;W, .380 ACP, 10 mm Auto, 5.7×28 mm, .22 Long Rifle; and shotgun calibers such as, for example, 12 gauge, 20 gauge, 28 gauge, .410 gauge, 10 gauge, 16 gauge. The illustrated embodiments focus on a lower receiver for the AR-15 variant (civilian) or M16/M4 (military) firearm platform (i.e., AR-15 style firearms); however, the concepts and features described herein can be are also applicable (with potential necessary alterations for particular applications) to other components of AR-15 style firearms and to components of other firearms. 
     In some cases, a firearm  1  includes a receiver assembly  100 , an upper receiver  10 , a charging handle  11 , a buffer tube  12 , a stock  13 , a grip  14 , a magazine  15 , and a bolt carrier group  16  (see  FIGS.  1 A and  1 B ). Other components, including, for example, a barrel, a fire control group, and a handguard, are not illustrated for simplicity. 
     According to certain embodiments of the present invention, as shown in  FIGS.  1 A- 1 B , the receiver assembly  100  may include a magazine release assembly  120 , a bolt release assembly  140 , and a safety selector assembly  160 . As shown in  FIG.  2 A , the receiver assembly  100  may also include a receiver body  101 , a threaded mount  102 , and a magazine well  105 . In some embodiments, the receiver assembly  100  interfaces with the upper receiver  10  with two pinned connections including an interface at a takedown pin hole  106  and at a pivot pin hole  107 . The takedown pin hole  106  may extend through both the receiver body  101  and the threaded mount  102 . The magazine  15  may be capable of being inserted into the magazine well  105  (see  FIGS.  1 A- 2 B ). In some embodiments, the magazine  15  is a Standardization Agreement (STANAG) magazine (designed for 5.56×45 mm NATO and/or .223 Remington ammunition), a magazine designed for 7.62×35 mm (.300 AAC Blackout), a SR-25 pattern magazine (designed for 7.62×51 mm NATO and/or .308 Winchester ammunition), a STANAG magazine designed for alternative calibers (e.g., pistol calibers including, for example, 9×19 mm, .45 ACP, .40 S&amp;W, .380 ACP, 10 mm Auto, 5.7×28 mm, .22 Long Rifle, etc.), or any other appropriate magazine. The grip  14  may attach to a grip interface portion  101   a  of the receiver body  101 . In some embodiments, the grip  14  may be an integral component of the receiver body  101 . 
     The threaded mount  102  may be an integral portion of the receiver body  101  (typical for metallic lower receivers) or may be a separate component (e.g., see exploded view in  FIG.  3   ). In some embodiments, the threaded mount  102  and the receiver body  101  are different materials. For example, the threaded mount  102  may be a metallic material and the receiver body  101  may be a non-metallic material, such as a polymer material, a plastic material, a composite material, or any appropriate non-metallic material. In some situations, the stress induced at the threaded connection  102   a  of the threaded mount  102  (i.e., cantilevered attachment of the buffer tube  12 ) is appropriate for a metallic component. Where the threaded mount  102  and the receiver body  101  are different materials, the receiver body  101  may be molded onto or around the threaded mount  102 . For example, the receiver body  101  may be co-molded or injection molded relative to the threaded mount  102 . One or both of the takedown pin hole  106  and the safety selector hole  119   a  may be used to locate the threaded mount  102  for the tooling (e.g., an injection molding machine). As shown in  FIG.  3   , the threaded mount  102  may include a plurality of retaining features  104  such that the material of the receiver body  101  can flow into or otherwise engage the retaining features  104  to ensure sufficient engagement between the threaded mount  102  and the receiver body  101 . The threaded mount  102  may also include a serial number plate  103  that protrudes through the receiver body  101  such that the serial number plate  103  is continuous with the outer surface of the receiver body  101  (see  FIGS.  1 B,  2 A, and  3   ). 
     For embodiments that include polymer materials for some portion(s) of the receiver assembly  100 , the polymer material may improve some characteristics of the firearm  1 . For example, compared to some metallic materials (such as aluminum), the polymer material may absorb and dissipate more energy and/or vibration. This results in less energy transferred from the chamber of the firearm (where the cartridge is fired) to the operator (i.e., less recoil). Consequently, after firing a round, the operator can more quickly acquire subsequent targets, which results in greater accuracy for additional shots fired. In other words, some of the energy from firing the cartridge is absorbed in receiver body  101  without being transferred to the operator (where conventional metallic receivers will transfer a greater percentage of the energy to the operator). 
     In some embodiments, the magazine release assembly  120  includes at least one mechanism for releasing the magazine  15  from the magazine well  105 . In particular, the magazine may be released due to movement of the left and/or right side magazine release portions  121 ,  122 . Conventional lower receivers include a button-operated mechanism that releases a magazine based on linear movement where the mechanism can only be operated from the right side of the firearm (designed exclusively for right-handed operators). While the magazine release assembly  120  may include a single mechanism on only one side of the firearm, in some embodiments, the magazine release assembly  120  includes a left side magazine release portion  121  and a right side magazine release portion  122  such that the magazine release assembly  120  is fully ambidextrous. In some embodiments, the left and/or right side magazine release portions  121 ,  122  may each include a lever mechanism (as described below with vertical pins  123 ,  124 ) while in other embodiments, the magazine release assembly  120  includes other modes of operation including, for example, electronic, gear-driven, belt-driven, linear actuators, other mechanical systems, or any other appropriate type of operation. In other words, the magazine release assembly  120  may include at least one pivoting lever. 
     As shown in  FIG.  2 A , the left side magazine release portion  121  may include a rear portion  121   a  closer to the firearm trigger such that the rear portion  121   a  is designed to interface with the operator’s left index finger (i.e., the left side magazine release portion  121  is designed for left-handed operators). The forward end of the left side magazine release portion  121  includes a protrusion  129  that extends through hole  108  of the receiver body  101  and engages a corresponding locking recess of the magazine  15  (see  FIG.  3   -5A). The protrusion  129  may be compatible with conventional magazines and/or various commercially available magazines including, for example, a Standardization Agreement (STANAG) magazine, designed for 5.56×45 mm NATO ammunition. As shown in  FIG.  3   -4B, the left side magazine release portion  121  may include an arm  125  such that the left side magazine release portion  121  is designed to pivot about a vertical axis defined by left side vertical pin  123 . The arm  125  may be inserted into passage  113  (see  FIG.  5 A ) and the pin  123  is engaged in a retaining feature  109  that is located above, below, or both above and below the passage  113 . In some embodiments, the retaining feature  109  is designed to provide a “snap-fit” such that the left side magazine release portion  121  and pin  123  can be pressed into position (and/or removed) relative to the receiver body  101  without any specialty tools. In some cases, to bias the left side magazine release portion  121  toward engagement with the magazine  15 , the magazine release assembly  120  includes a spring within hole  117  such that the spring interfaces with recess  127  of the left side magazine release portion  121  (see  FIG.  4 B ). The left side magazine release portion  121  is biased in the clockwise direction when viewed from above (by the spring in hole  117 ) such that protrusion  129  is biased toward engagement with the magazine and pressure on the rear portion  121   a  of the left side magazine release portion  121  (e.g., from the operator’s left index finger) will cause the left side magazine release portion  121  to rotate in the counter-clockwise direction (against spring pressure) to disengage the protrusion  129  from the corresponding feature of the magazine  15 . Rotation of the left side magazine release portion  121  also causes the arm  125  to rotate within the passage  113 . In some cases, there is a ball-nosed plunger at the end of the spring (in hole  117 ) for interfacing with the recess  127 . The spring and the ball-nosed plunger are not illustrated for simplicity. In some embodiments, the hole  117  is a through hole that extends through a full width of the receiver body  101 ; however, in other embodiments, each side of the receiver body  101  includes a separate hole that may or may not align with one another (i.e., there is a separate hole for the left side magazine release portion  121  and the right side magazine release portion  122 ). 
     Although the pin  123  is illustrated as a separate component from the left side magazine release portion  121 , in some cases, the pin  123  is an integral component of the left side magazine release portion  121 . In such a configuration, the left side magazine release portion  121  can be directly attached and/or detached from the retaining feature  109  without any intervening component. 
     As shown in  FIG.  2 B , the right side magazine release portion  122  includes a rear portion  122   a  adjacent to the firearm trigger such that the rear portion  122   a  is designed to interface with the operator’s right index finger (i.e., the right side magazine release portion  122  is designed for right-handed operators). The right side magazine release portion  122  does not extend as far forward as the left side magazine release portion  121  because the relevant feature (i.e., the locking recess of the magazine) is located on the left side. As shown in  FIG.  3   -4B, the right side magazine release portion  122  may include an arm  126  such that the right side magazine release portion  122  is designed to pivot about a vertical axis defined by right side vertical pin  124 . The arm  126  may be inserted into passage  113  (see  FIG.  5 B ) and the pin  124  is engaged in a retaining feature  110  that is located above, below, or both above and below the passage  113 . In some embodiments, the retaining feature  110  is designed to provide a “snap-fit” such that the right side magazine release portion  122  and pin  124  can be pressed into position (and/or removed) relative to the receiver body  101  without any specialty tools. In some cases, to bias the right side magazine release portion  122 , the magazine release assembly  120  includes a spring within hole  117  such that the spring interfaces with recess  128  of the right side magazine release portion  122  (see  FIGS.  3  and  4 A ). The right side magazine release portion  122  is biased in the counter-clockwise direction (by the spring in hole  117 ) such that pressure on the rear portion  122   a  of the right side magazine release portion  122  (e.g., from the operator’s right index finger) will cause the right side magazine release portion  122  to rotate in the clockwise direction when viewed from above (against spring pressure). Rotation of the right side magazine release portion  122  causes the arm  126  to rotate within the passage  113  such that the arm  126  presses against arm  125  causing the left side magazine release portion  121  to rotate in the counter-clockwise direction thus causing the protrusion  129  to disengage from the corresponding feature of the magazine  15 . In other words, in some cases, movement of the right side magazine release portion  122  causes movement of the left side magazine release portion  121 , which results in the disengagement of the protrusion  129  from the magazine  15  (when a magazine is present). In some cases, there is a ball-nosed plunger at the end of the spring (in hole  117 ) for interfacing with the recess  128 . The spring and the ball-nosed plunger are not illustrated for simplicity. As described above, in some examples, the hole  117  is common to both the left side magazine release portion  121  and the right side magazine release portion  122 , but this is not always the case. 
     Although the pin  124  is illustrated as a separate component from the right side magazine release portion  122 , in some cases, the pin  124  is an integral component of the right side magazine release portion  122 . In such a configuration, the right side magazine release portion  122  can be directly attached and/or detached from the retaining feature  110  without any intervening component. 
     In some cases, the operator interface portions (rear portion  121   a  and rear portion  122   a ) are symmetric on each side of the receiver assembly  100 . Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     The left and/or right side magazine release portions  121 ,  122  may be metallic components in some embodiments. In addition, the left and right side vertical pins  123 ,  124  may be metallic. In other embodiments, at least some portions of the left and right side magazine release portions  121 ,  122  (and/or the left and right side vertical pins  123 ,  124 ) may be a non-metallic material (e.g., polymer). 
     In some embodiments, the bolt release assembly  140  includes at least one mechanism for manipulating the bolt carrier group  16 . In some cases, the bolt carrier group  16  is biased toward a forward end of the firearm (e.g., by a spring within the buffer tube  12 ). In certain conditions, the bolt release assembly  140  engages and holds the bolt carrier group  16  in a rear position (see  FIG.  1 A ) where the rear surface  154  of the bolt release central portion  150  engages the forward face of the bolt carrier group  16 . The bolt release central portion  150  is at least partially located within the cavity  118  of the receiver body  101 , and the bolt release central portion  150  can be raised upward due to interface between the forward protrusion  153  and the follower of the magazine  15  or due to the left and/or right side bolt release portions  141 ,  142 . 
     Conventional lower receivers include a pivoting mechanism that manipulates a bolt carrier group based on rotational movement where the mechanism can only be operated from the left side of the firearm. While the bolt release assembly  140  may include a single mechanism on only one side of the firearm, in some embodiments, the bolt release assembly  140  includes a left side bolt release portion  141  and a right side bolt release portion  142  such that the bolt release assembly  140  is fully ambidextrous. In some embodiments, the left and/or right side bolt release portions  141 ,  142  may each include a lever mechanism (as described below with pins  143 ,  144 ) while in other embodiments, the bolt release assembly  140  includes other modes of operation including, for example, electronic, gear-driven, belt-driven, linear actuators, other mechanical systems, or any other appropriate type of operation. In other words, the bolt release assembly  140  may include at least one pivoting lever. 
     As shown in  FIGS.  4 A and  4 B , the left side bolt release portion  141  includes a lower portion  141   a  and an upper portion  141   b  (designed for an operator to manipulate the left side bolt release portion  141 ) along with an arm  145  and is designed to pivot about a forward/aft axis defined by left side pin  143 . In some embodiments, the arm  145  may be inserted into passage  114   a  (see  FIG.  5 A ) which intersects cavity  118 , and the pin  143  is engaged in a retaining feature  111  that is located forward, aft, or both forward and aft of the passage  114   a . The arm  145  engages a front cutout  151  of the bolt release central portion  150 . In some embodiments, the retaining feature  111  is designed to provide a “snap-fit” such that the left side bolt release portion  141  and pin  143  can be pressed into position (and/or removed) relative to the receiver body  101  without any specialty tools. Pressing the lower portion  141   a  causes the left side bolt release portion  141  to rotate about the left side pin  143  such that the arm  145  pivots and causes the bolt release central portion  150  to move upward (i.e., toward a position where the bolt release central portion  150  would engage the bolt carrier group  16 ). Similarly, pressing the upper portion  141   b  causes the left side bolt release portion  141  to rotate about the left side pin  143  such that the arm  145  pivots and causes the bolt release central portion  150  to move downward (i.e., away from a position where the bolt release central portion  150  would engage the bolt carrier group  16 ). In some cases, the bolt release assembly  140  includes a spring within hole  115  such that the spring interfaces with recess  147  of the left side bolt release portion  141  (see  FIG.  4 B ) to bias the bolt release central portion  150  downward such that the bolt carrier group  16  can move past the bolt release assembly  140 . In some embodiments, the bolt release central portion  150  translates or moves linearly (i.e., does not rotate). 
     Although the pin  143  is illustrated as a separate component from the left side bolt release portion  141 , in some cases, the pin  143  is an integral component of the left side bolt release portion  141 . In such a configuration, the left side bolt release portion  141  can be directly attached and/or detached from the retaining feature  111  without any intervening component. 
     As shown in  FIGS.  4 A and  4 B , the right side bolt release portion  142  includes a lower portion  142   a  and an upper portion  142   b  (designed for an operator to manipulate the right side bolt release portion  142 ) along with an arm  146  and is designed to pivot about a forward/aft axis defined by right side pin  144 . In some embodiments, the arm  146  may be inserted into passage  114   b  (see  FIG.  5 B ) which intersects cavity  118 , and the pin  144  is engaged in a retaining feature  112  that is located forward, aft, or both forward and aft of the passage  114   b . The arm  146  engages a rear cutout  152  of the bolt release central portion  150 . In some embodiments, the retaining feature  112  is designed to provide a “snap-fit” such that the right side bolt release portion  142  and pin  144  can be pressed into position (and/or removed) relative to the receiver body  101  without any specialty tools. Pressing the lower portion  142   a  causes the right side bolt release portion  142  to rotate about the right side pin  144  such that the arm  146  pivots and causes the bolt release central portion  150  to move upward (i.e., toward a position where the bolt release central portion  150  would engage the bolt carrier group  16 ). Similarly, pressing the upper portion  142   b  causes the right side bolt release portion  142  to rotate about the left side pin  144  such that the arm  146  pivots and causes the bolt release central portion  150  to move downward (i.e., away from a position where the bolt release central portion  150  would engage the bolt carrier group  16 ). In some cases, the bolt release assembly  140  includes a spring within hole  116  such that the spring interfaces with recess  148  of the right side bolt release portion  142  (see  FIG.  4 A ) to bias the bolt release central portion  150  downward such that the bolt carrier group  16  can move past the bolt release assembly  140 . Accordingly, the bolt release assembly  140  may include two springs acting together to bias the bolt release central portion  150  downward (i.e., one spring interfacing with recess  147  of the left side bolt release portion  141  and a second spring interfacing with recess  148  of the right side bolt release portion  142 ). In some embodiments, the bolt release central portion  150  translates or moves linearly (i.e., does not rotate). 
     Although the pin  144  is illustrated as a separate component from the right side bolt release portion  142 , in some cases, the pin  144  is an integral component of the right side bolt release portion  142 . In such a configuration, the right side bolt release portion  142  can be directly attached and/or detached from the retaining feature  112  without any intervening component. 
     Based on the movement of at least one of arms  145 ,  146  (as described above), and/or the follower of the magazine  15  pushing on forward protrusion  153 , the bolt release central portion  150  moves approximately vertically within cavity  118 . In other words, the bolt release central portion  150  translates approximately vertically (i.e., linearly) within cavity  118 , while conventional bolt release mechanisms pivot (without translating). 
     In some cases, the operator interface portions for raising the bolt release central portion  150  (lower portion  141   a  and lower portion  142   a ) are symmetric on each side of the receiver assembly  100 . Similarly, the operator interface portions for lowering the bolt release central portion  150  (upper portion  141   b  and upper portion  142   b ) are symmetric on each side of the receiver assembly  100 . Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     The left and/or right side bolt release portions  141 ,  142  may be metallic components in some embodiments. In addition, the left and right side pins  143 ,  144  may be metallic. In other embodiments, at least some portions of the left and right side bolt release portions  141 ,  142  (and/or the left and right side pins  143 ,  144 ) may be a non-metallic material (e.g., polymer). 
     As shown in  FIG.  1 A -3, the safety selector assembly  160  may interface with the safety selector hole  119   a . The safety selector assembly  160  includes at least one safety portion, and, in some cases, includes a left side safety portion  161  and a right side safety portion  162  such that the safety selector assembly  160  is fully ambidextrous. In addition to the left side safety portion  161  and the right side safety portion  162 , the safety selector assembly  160  (see  FIGS.  6 A and  6 B ) may include a selector shaft  181 , a selector cap  182 , a detent clip  170 , and at least one fastener  198 ,  199 . Conventional safety assemblies include a detent and spring that pass through a vertical hole in the receiver body  101  that aligns with the pistol grip  14 . The safety selector assembly  160  is compatible with typical detent and spring arrangements where the detent interfaces with the radial slot  192  of the selector shaft  181 . However, the safety selector assembly  160  may include an improved arrangement for constraining and dictating motion for the safety assembly (i.e., the assembly can function without the typical detent and spring). At the right end of the safety selector assembly  160 , a first end of the selector shaft  181  has an outer diameter that approximately matches the inner diameter of the safety selector hole  119   a . The first end may also include a recess  183  that approximately matches the protrusion  166  of the right side safety portion  162  such that when the right side safety portion  162  and the selector shaft  181  are rotationally constrained with one another. The fastener  199  passes through hole  169  of the right side safety portion  162  and threads into hole  169  at the right end of the selector shaft  181 . 
     At the left end of the safety selector assembly  160 , the selector cap  182  has an outer diameter that approximately matches the inner diameter of the safety selector hole  119   a . The outer face of the selector cap  182  may also include a recess  184  that approximately matches the protrusion on the inner surface of the left side safety portion  161  (similar to the protrusion  166  of the right side safety portion  162 ) such that the left side safety portion  161 , the selector cap  182 , and the selector shaft  181  are rotationally constrained with one another. The fastener  198  passes through hole  169  of the left side safety portion  161  and through hole  169  of the selector cap  182  before threading into hole  169  at the left end of the selector shaft  181 . 
     In some embodiments, the left end of the selector shaft  181  includes at least one protrusion that extends from surface  189   a . The protrusion may have any appropriate shape including, but not limited to, cylindrical. The protrusion may facilitate the assembly process such that the protrusion limits movement of the first arm  173  and/or the second arm  174  relative to the selector shaft  181 . 
     The selector cap  182  and the selector shaft  181  interface with one another near the inner surface  102   b  of the threaded mount  102 . The selector cap  182  includes at least one protrusion  187  where the protrusion(s)  187  engage the open portions  185 ,  186  at the left end of the selector shaft  181 . In addition, the detent clip  170  is arranged against the inner surface  102   b  of the threaded mount  102  such that the protrusion  171  engages the inner hole  119   b  adjacent to the safety selector hole  119   a  and the arch section  172  engages the outer surface  181   a  at the left end of the selector shaft  181 . In the assembled state, the detent clip  170  is sandwiched between (i) the end surface  189  of the selector shaft  181  and (ii) the inner surface  102   b  of the threaded mount  102 . As shown in  FIGS.  6 B and  6 E , the detent clip  170  includes a first arm  173  and a second arm  174 . The first arm  173  includes a first protrusion  175  where the first protrusion  175  has an inner surface  175   a  and an outer surface  175   b . The second arm  174  includes a second protrusion  176  where the second protrusion  176  has an inner surface  176   a  and an outer surface  176   b . 
     The left side safety portion  161  includes a finger interface portion  163  and an indicator protrusion  167  that points toward a symbol or other indicator (e.g., text) on the surface of the receiver body  101 . In some cases, the indicator protrusion  167  points toward the forward end of the firearm (see  FIGS.  1 A- 2 B,  6 A, and  6 B ) when the safety selector assembly  160  is in the safe condition. Similarly, the right side safety portion  162  includes a finger interface portion  164  and an indicator protrusion  168  that points toward a symbol or other indicator (e.g., text) on the surface of the receiver body  101 . In some cases, the indicator protrusion  168  points toward the forward end of the firearm (see  FIGS.  1 A- 2 B,  6 A, and  6 B ) when the safety selector assembly  160  is in the safe condition. In some cases, the indicator protrusion  167  points toward the top of the firearm when the safety selector assembly  160  is in the fire condition. Similarly, the indicator protrusion  168  may point toward the top of the firearm when the safety selector assembly  160  is in the fire condition. 
     In some cases, the operator interface portions (finger interface portion  163  and finger interface portion  164 ) are symmetric on each side of the receiver assembly  100 . Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     In some embodiments, the interface between the safety selector assembly  160  and the fire control group (and the resultant condition of the safety selector assembly  160 ) is based on rotation of the selector shaft  181 . For a description of the function of a fire control group, see U.S. Pat. No. 10,670,360, which is hereby incorporated in its entirety by this reference. When the cylindrical surface  190  of the selector shaft  181  (see  FIGS.  6 F and  6 G ) faces downward (i.e., toward the grip interface portion  101   a ), the safety selector assembly  160  prevents rotation of the trigger (i.e., safe condition). When the selector shaft  181  is rotated such that the planar surface  191  of the selector shaft  181  (see  FIGS.  6 F and  6 G ) faces downward (i.e., toward the grip interface portion  101   a ), the safety selector assembly  160  allows rotation of the trigger (i.e., fire condition). 
     When the safety selector assembly  160  is in the safe condition, the first protrusion  175  of the first arm  173  engages the open portion  185  of the selector shaft  181  (see  FIGS.  6 A and  6 B ). The first protrusion  175  acts as a detent because the detent clip  170  functions as a spring where the first arm  173  and the second arm  174  are biased toward converging inward to one another. The detent clip  170  cannot rotate with the selector shaft  181  because the protrusion  171  engages the inner hole  119   b . Based on the geometry of the first protrusion  175 , the safety selector assembly  160  can only rotate one direction from the safe condition. If the operator tries to rotate the safety selector assembly  160  to point the indicator protrusions  167 ,  168  downward (i.e., toward the grip interface portion  101   a ), the outer surface  175   b  engages the adjacent inner surface of the open portion  185  and thus prevents rotation of the safety selector assembly  160 . If the operator tries to rotate the safety selector assembly  160  to point the indicator protrusions  167 ,  168  upward (i.e., away from the grip interface portion  101   a ), rotation of the safety selector assembly  160  causes the inner surface  175   a  to press against the adjacent inner surface of the open portion  185 . Based on the geometry of the inner surface  175   a , the interface between the inner surface of the open portion  185  and the inner surface  175   a  causes the first arm  173  to deflect away from the selector shaft  181  thus allowing rotation of the safety selector assembly  160 . The safety selector assembly  160  can rotate approximately 90° until the second protrusion  176  (of the second arm  174 ) engages the open portion  186 . Like the first protrusion  175 , the second protrusion  176  acts as a detent because the second arm  174  is biased toward engaging the open portion  186 . When the second protrusion  176  engages the open portion  186 , the safety selector assembly  160  is in the fire condition. 
     When the safety selector assembly  160  is in the fire condition, the second protrusion  176  of the second arm  174  engages the open portion  186 . The second protrusion  176  acts as a detent because the detent clip  170  functions as a spring where the first arm  173  and the second arm  174  are biased toward converging towards one another. The detent clip  170  cannot rotate with the selector shaft  181  because the protrusion  171  engages the inner hole  119   b . Based on the geometry of the second protrusion  176 , the safety selector assembly  160  can only rotate one direction from the fire condition. If the operator tries to rotate the safety selector assembly  160  to point the indicator protrusions  167 ,  168  rearward (i.e., toward the threaded mount  102 ), the outer surface  176   b  engages the adjacent inner surface of the open portion  186  and thus prevents rotation of the safety selector assembly  160 . If the operator tries to rotate the safety selector assembly  160  to point the indicator protrusions  167 ,  168  forward (i.e., toward the magazine  15 ), the inner surface  176   a  presses against the adjacent inner surface of the open portion  186 . Based on the geometry of the inner surface  176   a , the interface between the inner surface of the open portion  186  and the inner surface  176   a  causes the second arm  174  to deflect away from the selector shaft  181  thus allowing rotation of the safety selector assembly  160 . The safety selector assembly  160  can rotate approximately 90° until the second protrusion  175  engages the open portion  185  (i.e., the safe condition described above). 
     In some embodiments, the threaded mount  102  includes provisions for safety selector components that extend upward from the grip interface portion  101   a . For example, as shown in  FIG.  16   -17B, the threaded mount  102  may include a hole  119   c  that extends in an approximately vertical direction where a lower end of the hole  119   c  extends to a lower surface  101   b  of the receiver body  101 . The upper end of hole  119   c  may extend to and/or intersect with the safety selector hole  119   a . As shown in  FIG.  16   , the threaded mount  102  may include a lower surface  119   e  that is approximately continuous and/or coplanar with the lower surface  101   b  of the receiver body  101 . The threaded mount  102  may include a protrusion that extends in at least one direction in the area adjacent to hole  119   c . For example, as shown in  FIGS.  17 A and  17 B , the threaded mount  102  may include a protrusion in the lateral direction (inward) and vertically (down). In some embodiments, a detent and a spring are inserted into hole  119   c  from the bottom and are held in position by the grip  14 . The detent and spring are not shown for clarity. In some embodiments, as shown in  FIG.  18   , the threaded mount  102  includes a gap  119   f  in the area below the safety selector hole  119   a . The gap  119   f  allows a feature (i.e., a hole for the detent and spring) to be molded into the receiver body  101  or a hole may be drilled into a portion of the receiver body  101  after the molding process. 
     As shown in  FIGS.  7 A and  7 B , in some cases, a firearm  1  includes a receiver assembly  200 , an upper receiver  10 , a charging handle  11 , a buffer tube  12 , a stock  13 , a grip  14 , a magazine  17 , and a bolt carrier group  16 . Other components, including, for example, a barrel, a fire control group, and a handguard, are not illustrated for simplicity. 
     According to certain embodiments of the present invention, the receiver assembly  200  may include a magazine release assembly  220 , a bolt release assembly  240 , and a safety selector assembly  260 . As shown in  FIG.  8 A , the receiver assembly  200  may also include a receiver body  201 , a threaded mount  202 , and a magazine well  205 . In some embodiments, the receiver assembly  200  interfaces with the upper receiver  10  with two pinned connections including an interface at a takedown pin hole  206  and at a pivot pin hole  207 . The takedown pin hole  206  may extend through both the receiver body  201  and the threaded mount  202 . The magazine  17  may be capable of being inserted into the magazine well  205  (see  FIGS.  7 A- 8 B ). In some embodiments, the magazine  17  is a commercially available magazine designed for handguns (e.g., pistol calibers including, for example, 9×19 mm, .45 ACP, .40, S&amp;W, .380 ACP, 10 mm Auto, 5.7×28 mm, .22 Long Rifle, etc.) or any other appropriate magazine. The magazine  17  may be a standard magazine designed for rifle caliber (e.g., 5.56×45 mm NATO and/or .223 Remington, 7.62×35 mm or .300 AAC Blackout, 7.62×51 mm NATO and/or .308 Winchester ammunition, etc. The grip  14  may attach to a grip interface portion  201   a  of the receiver body  201 . In some embodiments, the grip  14  may be an integral component of the receiver body  201 . 
     The threaded mount  202  may be an integral portion of the receiver body  201  (typical for metallic lower receivers) or may be a separate component (e.g., see exploded view in  FIG.  3   ). In some embodiments, the threaded mount  202  and the receiver body  201  are different materials. For example, the threaded mount  202  may be a metallic material and the receiver body  201  may be a non-metallic material, such as a polymer material, a plastic material, a composite material, or any appropriate non-metallic material. In some situations, the stress induced at the threaded connection  202   a  of the threaded mount  202  (i.e., cantilevered attachment of the buffer tube  12 ) is appropriate for a metallic component. Where the threaded mount  202  and the receiver body  201  are different materials, the receiver body  201  may be molded onto or around the threaded mount  202 . For example, the receiver body  201  may be co-molded or injection molded relative to the threaded mount  202 . One or both of the takedown pin hole  206  and the safety selector hole  219   a  may be used to locate the threaded mount  202  for the tooling (e.g., an injection molding machine). The threaded mount  202  may include a plurality of retaining features such that the material of the receiver body  201  can flow into or otherwise engage the retaining features to ensure sufficient engagement between the threaded mount  202  and the receiver body  201  (see, e.g., retaining features  104  in  FIG.  3   ). The threaded mount  202  may also include a serial number plate  203  that protrudes through the receiver body  201  such that the serial number plate  203  is continuous with the outer surface of the receiver body  201  (see  FIGS.  7 B and  8 A ). 
     For embodiments that include polymer materials for some portion(s) of the receiver assembly  200 , the polymer material may improve some characteristics of the firearm  1 . For example, compared to some metallic materials (such as aluminum), the polymer material may absorb and dissipate more energy and/or vibration. This results in less energy transferred from the chamber of the firearm (where the cartridge is fired) to the operator (i.e., less recoil). Consequently, after firing a round, the operator can more quickly acquire subsequent targets, which results in greater accuracy for additional shots fired. In other words, some of the energy from firing the cartridge is absorbed in receiver body  201  without being transferred to the operator (where conventional metallic receivers will transfer a greater percentage of the energy to the operator). 
     As shown in  FIG.  9   , in some embodiments, the receiver body  201  includes at least one rib  201   c  (and/or channel). The rib(s)  201   c  may be located in the magazine well  205 . In some cases, the rib(s)  201   c  help expel dirt and foreign objects from the magazine well  205 , lighten the receiver body  201 , reduce thickness in the relevant areas of the receiver body  201 , reduce manufacturing cycle time for the receiver body  201 , and/or more improve efficient heat transfer of the receiver body  201 . 
     In some embodiments, the magazine release assembly  220  includes at least one mechanism for releasing the magazine  17  from the magazine well  205 . In particular, the magazine may be released due to movement of the left and/or right side magazine release portions  221 ,  222 . Conventional lower receivers include a button-operated mechanism that releases a magazine based on linear movement where the mechanism can only be operated from the right side of the firearm (designed exclusively for right-handed operators). While the magazine release assembly  220  may include a single mechanism on only one side of the firearm, in some embodiments, the magazine release assembly  220  includes a left side magazine release portion  221  and a right side magazine release portion  222  such that the magazine release assembly  220  is fully ambidextrous (see  FIGS.  8 A,  8 B,  11 A, and  11 B ). In some embodiments, the left and/or right side magazine release portions  221 ,  222  may each include a lever mechanism (as described below with vertical pins  223 ,  224 ) while in other embodiments, the magazine release assembly  220  includes other modes of operation including, for example, electronic, gear-driven, belt-driven, linear actuators, other mechanical systems, or any other appropriate type of operation. In other words, the magazine release assembly  220  may include at least one pivoting lever. 
     As shown in  FIG.  8 A , the left side magazine release portion  221  may include a rear portion  221   a  closer to the firearm trigger such that the rear portion  221   a  is designed to interface with the operator’s left index finger (i.e., the left side magazine release portion  221  is designed for left-handed operators). The left side magazine release portion  221  does not extend as far forward as the right side magazine release portion  222  because the relevant feature (i.e., the locking recess of the magazine  17 ) may be located on the right side. As shown in  FIGS.  11 A- 11 B , the left side magazine release portion  221  may include an arm  225  such that the left side magazine release portion  221  is designed to pivot about a vertical axis defined by left side vertical pin  223 . The arm  225  may be inserted into passage  213  (see  FIG.  13 A ) and the pin  223  is engaged in a retaining feature  209  that is located above, below, or both above and below the passage  213 . In some embodiments, the retaining feature  209  is a hole extending down from an upper surface of the receiver body  201  (see  FIGS.  13 A- 13 B ) and the vertical pin  223  is a set screw that may include threads  223 . 1 . In some cases, the threads  223 . 1  are disposed at the upper end of the pin  223 , and the threads  223 . 1  engage the hole  209 . In other embodiments, the retaining feature  209  is designed to provide a “snap-fit” such that the left side magazine release portion  221  and pin  223  can be pressed into position (and/or removed) relative to the receiver body  201  without any specialty tools. In some cases, to bias the left side magazine release portion  221  toward engagement with the magazine  17 , the magazine release assembly  220  includes a spring within hole  217  such that the spring interfaces with recess  227  of the left side magazine release portion  221  (see  FIG.  11 A ). The left side magazine release portion  221  is biased in the clockwise direction when viewed from above (by the spring in hole  217 ) such that pressure on the rear portion  221   a  of the left side magazine release portion  221  (e.g., from the operator’s left index finger) will cause the left side magazine release portion  221  to rotate in the counter-clockwise direction when viewed from above (against spring pressure). Rotation of the left side magazine release portion  221  also causes the arm  225  to rotate within the passage  213 . Rotation of the left side magazine release portion  221  causes the arm  225  to rotate within the passage  213  such that the arm  225  presses against arm  226  causing the right side magazine release portion  222  to rotate in the clockwise direction (when viewed from above) thus causing the protrusion  229  to disengage from the corresponding feature of the magazine  17 . In other words, in some cases, movement of the left side magazine release portion  221  causes movement of the right side magazine release portion  222 , which results in the disengagement of the protrusion  229  from the magazine  17  (when a magazine is present). In some cases, there is a ball-nosed plunger at the end of the spring (in hole  217 ) for interfacing with the recess  227 . The spring and the ball-nosed plunger are not illustrated for simplicity. In some embodiments, the hole  217  is a through hole that extends through a full width of the receiver body  201 ; however, in other embodiments, each side of the receiver body  201  includes a separate hole that may or may not align with one another (i.e., there is a separate hole for the left side magazine release portion  221  and the right side magazine release portion  222 ). 
     As shown in  FIG.  8 B , the right side magazine release portion  222  includes a rear portion  222   a  adjacent to the firearm trigger such that the rear portion  222   a  is designed to interface with the operator’s right index finger (i.e., the right side magazine release portion  222  is designed for right-handed operators). The forward end of the right side magazine release portion  222  includes a protrusion  229  that extends through hole  208  of the receiver body  201  and engages a corresponding locking recess of the magazine  17  (see  FIGS.  8 B and  10 B- 11 B ). The protrusion  229  may be compatible with conventional magazines and/or various commercially available magazines including, for example, pistol magazines designed for a pistol caliber (e.g., pistol calibers including, for example, 9×19mm, .45 ACP, .40 S&amp;W, .380 ACP, 10 mm Auto, 5.7×28 mm, .22 Long Rifle, etc.). As shown in  FIGS.  8 B and  10 B- 11 B , the right side magazine release portion  222  may include an arm  226  such that the right side magazine release portion  222  is designed to pivot about a vertical axis defined by right side vertical pin  224 . The arm  226  may be inserted into passage  213  (see  FIG.  13 B ) and the pin  224  is engaged in a retaining feature  210  that is located above, below, or both above and below the passage  213 . In some embodiments, the retaining feature  210  is a hole extending down from an upper surface of the receiver body  201  (see  FIGS.  13 A- 13 B ) and the vertical pin  224  is a set screw that may include threads  224 . 1 . In some cases, the threads  224 . 1  are disposed at the upper end of the pin  224 , and the threads  224 . 1  engage the hole  210 . In other embodiments, the retaining feature  210  is designed to provide a “snap-fit” such that the right side magazine release portion  222  and pin  224  can be pressed into position (and/or removed) relative to the receiver body  201  without any specialty tools. In some cases, to bias the right side magazine release portion  222 , the magazine release assembly  220  includes a spring within hole  217  such that the spring interfaces with recess  228  of the right side magazine release portion  222  (see  FIG.  11 B ). The right side magazine release portion  222  is biased in the counter-clockwise direction when viewed from above (by the spring in hole  217 ) such that protrusion  229  is biased toward engagement with the magazine and pressure on the rear portion  222   a  of the right side magazine release portion  222  (e.g., from the operator’s right index finger) will cause the right side magazine release portion  222  to rotate in the clockwise direction when viewed from above to disengage the protrusion  229  from the corresponding feature of the magazine  17  (against spring pressure). Rotation of the right side magazine release portion  222  causes the arm  226  to rotate within the passage  213  such that the arm  226  presses against arm  225  causing the left side magazine release portion  221  to rotate in the counter-clockwise direction. In other words, in some cases, movement of the right side magazine release portion  222  causes movement of the left side magazine release portion  221 . In some cases, there is a ball-nosed plunger at the end of the spring (in hole  217 ) for interfacing with the recess  228 . The spring and the ball-nosed plunger are not illustrated for simplicity. As described above, in some examples, the hole  217  is common to both the left side magazine release portion  221  and the right side magazine release portion  222 , but this is not always the case. 
     In some cases, the operator interface portions (rear portion  221   a  and rear portion  222   a ) are symmetric on each side of the receiver assembly  200 . Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     As shown in  FIGS.  11 A and  11 B , in some embodiments, left and/or right side magazine release portions  221 ,  222  may each be a single unitary component (including the relevant arm  225 ,  226 ). In other embodiments, as shown in  FIG.  12 B , the magazine release assembly  220  may be arranged such that the left side magazine release portion  221  is a separate component from the arm  225  and the right side magazine release portion  222  is a separate component from the arm  226 . In some embodiments, the arm  225  includes a feature (e.g., feature  225 . 1 ) for engaging the left side magazine release portion  221  and/or the arm  226  includes a feature (e.g., feature  226 . 1 ) for engaging the right side magazine release portion  222 . In some embodiments, the features  225 . 1 ,  226 . 1  include a dovetail that engages a corresponding feature in the magazine release portion  221 ,  222 . In addition to a corresponding features for engaging feature  226 . 1 , the right side magazine release portion  222  may include a hole  222 . 1  such that insertion of the pin  224  secures the right side magazine release portion  222  relative to the arm  226 . In other words, insertion of the pin  224  dictates that the right side magazine release portion  222  cannot be disengaged from the arm  226 . The right side magazine release portion  222  may include a hole such that a set screw can be threaded through and at least partially into the arm  226  to secure the components together (either in addition to or in lieu of the engagement at hole  222 . 1 ). The left side magazine release portion  221  may include a similar hole  221 . 1  such that insertion of the pin  223  dictates that the left side magazine release portion  221  cannot be disengaged from the arm  225 . The left side magazine release portion  221  may include a hole such that a set screw can be threaded through and at least partially into the arm  225  to secure the components together (either in addition to or in lieu of the engagement at hole  221 . 1 ). In some embodiments, separation of the magazine release portions  221 ,  222  from the arms  225 ,  226  allows an operator to swap the external interfacing components (e.g., to change the color, texture, shape, size, and/or other characteristics of the left and/or right side magazine release portions  221 ,  222 ). 
     The left and/or right side magazine release portions  221 ,  222  may be metallic components in some embodiments. In addition, the left and right side vertical pins  223 ,  224  may be metallic. In other embodiments, at least some portions of the left and right side magazine release portions  221 ,  222  (and/or the left and right side vertical pins  223 ,  224 ) may be a non-metallic material (e.g., polymer). 
     In some embodiments, the bolt release assembly  240  includes at least one mechanism for manipulating the bolt carrier group  16 . In some cases, the bolt carrier group  16  is biased toward a forward end of the firearm (e.g., by a spring within the buffer tube  12 ). In certain conditions, the bolt release assembly  240  engages and holds the bolt carrier group  16  in a rear position (see  FIG.  7 A ) where the rear surface  254  of the bolt release central portion  250  engages the forward face of the bolt carrier group  16 . The bolt release central portion  250  is at least partially located within the cavity  218  of the receiver body  201 , and the bolt release central portion  250  can be raised upward due to upward movement of the follower of the magazine  17 . In some embodiments, a lifter  276  interfaces with the follower of the magazine  17  such that, when the magazine  17  is empty, the follower pushes upper end  278  upward (see  FIG.  8 A -9, 11A, 15A, and 15B). Upward movement of the lifter  276  causes portion  277  to interface with rocker  271  such that first end  272  is lifted upward by portion  277 . As shown in  FIG.  15 A , the portion  277  may be a step in some embodiments. In other embodiments, the portion  277  is an opening or aperture, as shown in  FIG.  15 B . Upward movement of first end  272  causes rocker  271  to rotate about pin  275 . The rotation of rocker  271  causes second end  273  to move downward. Downward movement of the second end  273  of the rocker  271  leads to an interface between the second end  273  and a forward end  256  of the long rocker  255  (which causes downward movement of the forward end  256 ). The long rocker  255  is shown in  FIGS.  8 A,  9 ,  11 A,  11 B, and  14   . Downward movement of the forward end  256  of the long rocker  255  causes the long rocker  255  to rotate about hole  258 . In some embodiments, the hole  258  engages protrusion  201 . 1  of the receiver body  201  (see  FIGS.  8 A,  10 A, and  13 A ). The rotation of long rocker  255  about hole  258  (caused by downward motion of the second end  273  and the interface with forward end  256 ) causes the rear end  257  to move upward. Upward movement of the rear end  257  of the long rocker  255  leads to an interface between the rear end  257  and a bolt release pin  270  such that the bolt release pin  270  is moved upward. The upward movement of the bolt release pin  270  causes the bolt release central portion  250  to move upward to engage the bolt carrier group  16  (in the rear position). In other words, movement of the follower of the magazine  17  and/or movement of the left and/or right side bolt release portions  241 ,  242  (as described below) can cause the bolt release central portion  250  to hold the bolt carrier group  16  in the rear position. In some embodiments, the bolt release central portion  250  translates or moves linearly (i.e., does not rotate). 
     Conventional lower receivers include a pivoting mechanism that manipulates a bolt carrier group based on rotational movement where the mechanism can only be operated from the left side of the firearm. While the bolt release assembly  240  may include a single mechanism on only one side of the firearm, in some embodiments, the bolt release assembly  240  includes a left side bolt release portion  241  and a right side bolt release portion  242  such that the bolt release assembly  240  is fully ambidextrous. In some embodiments, the left and/or right side bolt release portions  241 ,  242  may each include a lever mechanism (as described below with pins  243 ,  244 ) while in other embodiments, the bolt release assembly  240  includes other modes of operation including, for example, electronic, gear-driven, belt-driven, linear actuators, other mechanical systems, or any other appropriate type of operation. In other words, the bolt release assembly  240  may include at least one pivoting lever. 
     As shown in  FIGS.  11 A and  11 B , the left side bolt release portion  241  includes a lower portion  241   a  and an upper portion  241   b  (designed for an operator to manipulate the left side bolt release portion  241 ) along with an arm  245  and is designed to pivot about a forward/aft axis defined by left side pin  243 . In some embodiments, the arm  245  may be inserted into passage  214   a  (see  FIG.  13 A ) which intersects cavity  218 , and the pin  243  is engaged in a retaining feature  211  that is located forward, aft, or both forward and aft of the passage  214   a . The arm  245  may engage a first cutout  251  of the bolt release central portion  250 . In some embodiments, the retaining feature  211  is a hole extending fore/aft in the receiver body  201  (see  FIG.  13 A ) and the pin  243  is a set screw that may include threads  243 . 1 . In some cases, the threads  243 . 1  are disposed at the rear end of the pin  243 , and the threads  243 . 1  engage the hole  211 . In other embodiments, the retaining feature  211  is designed to provide a “snap-fit” such that the left side bolt release portion  241  and pin  243  can be pressed into position (and/or removed) relative to the receiver body  201  without any specialty tools. Pressing the lower portion  241   a  causes the left side bolt release portion  241  to rotate about the left side pin  243  such that the arm  245  pivots and causes the bolt release central portion  250  to move upward (i.e., toward a position where the bolt release central portion  250  would engage the bolt carrier group  16 ). Similarly, pressing the upper portion  241   b  causes the left side bolt release portion  241  to rotate about the left side pin  243  such that the arm  245  pivots and causes the bolt release central portion  250  to move downward (i.e., away from a position where the bolt release central portion  250  would engage the bolt carrier group  16 ). In some cases, the bolt release assembly  240  includes a spring within hole  215  such that the spring interfaces with recess  247  of the left side bolt release portion  241  (see  FIG.  11 A ) to bias the bolt release central portion  250  downward such that the bolt carrier group  16  can move past the bolt release assembly  240 . In some embodiments, the bolt release central portion  250  translates or moves linearly (i.e., does not rotate). 
     Although the pin  243  is illustrated as a separate component from the left side bolt release portion  241 , in some cases, the pin  243  is an integral component of the left side bolt release portion  241 . In such a configuration, the left side bolt release portion  241  can be directly attached and/or detached from the retaining feature  211  without any intervening component. 
     As shown in  FIGS.  11 A and  11 B , the right side bolt release portion  242  includes a lower portion  242   a  and an upper portion  242   b  (designed for an operator to manipulate the right side bolt release portion  242 ) along with an arm  246  and is designed to pivot about a forward/aft axis defined by right side pin  244 . In some embodiments, the arm  246  may be inserted into passage  214   b  (see  FIG.  13 B ) which intersects cavity  218 , and the pin  244  is engaged in a retaining feature  212  that is located forward, aft, or both forward and aft of the passage  214   b . The arm  246  engages a second cutout  252  of the bolt release central portion  250 . In some embodiments, the retaining feature  212  is a hole extending fore/aft in the receiver body  201  (see  FIG.  13 B ) and the pin  244  is a set screw that may include threads  244 . 1 . In some cases, the threads  244 . 1  are disposed at the rear end of the pin  244 , and the threads  244 . 1  engage the hole  212 . In other embodiments, the retaining feature  212  is designed to provide a “snap-fit” such that the right side bolt release portion  242  and pin  244  can be pressed into position (and/or removed) relative to the receiver body  201  without any specialty tools. Pressing the lower portion  242   a  causes the right side bolt release portion  242  to rotate about the right side pin  244  such that the arm  246  pivots and causes the bolt release central portion  250  to move upward (i.e., toward a position where the bolt release central portion  250  would engage the bolt carrier group  16 ). Similarly, pressing the upper portion  242   b  causes the right side bolt release portion  242  to rotate about the left side pin  244  such that the arm  246  pivots and causes the bolt release central portion  250  to move downward (i.e., away from a position where the bolt release central portion  250  would engage the bolt carrier group  16 ). In some cases, the bolt release assembly  240  includes a spring within hole  216  such that the spring interfaces with recess  248  of the right side bolt release portion  242  (see  FIG.  13 B ) to bias the bolt release central portion  250  downward such that the bolt carrier group  16  can move past the bolt release assembly  240 . Accordingly, the bolt release assembly  240  may include two springs acting together to bias the bolt release central portion  250  downward (i.e., one spring interfacing with recess  247  of the left side bolt release portion  241  and a second spring interfacing with recess  248  of the right side bolt release portion  242 ). 
     Although the pin  244  is illustrated as a separate component from the right side bolt release portion  242 , in some cases, the pin  244  is an integral component of the right side bolt release portion  242 . In such a configuration, the right side bolt release portion  242  can be directly attached and/or detached from the retaining feature  212  without any intervening component. 
     Based on the movement of at least one of arms  245 ,  246  (as described above), and/or the follower of the magazine  17  pushing on lifter  276  (as described above), the bolt release central portion  250  moves approximately vertically within cavity  218 . In other words, the bolt release central portion  250  translates approximately vertically (i.e., linearly) within cavity  218 , while conventional bolt release mechanisms pivot (without translating). 
     In some cases, the operator interface portions for raising the bolt release central portion  250  (lower portion  241   a  and lower portion  242   a ) are symmetric on each side of the receiver assembly  200 . Similarly, the operator interface portions for lowering the bolt release central portion  250  (upper portion  241   b  and upper portion  242   b ) may be symmetric on each side of the receiver assembly  200 . Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     As shown in  FIGS.  9 ,  11 A, and  11 B , in some embodiments, left and/or right side bolt release portions  241 ,  242  may each be a single unitary component (including the relevant arm  245 ,  246 ). In other embodiments, as shown in  FIG.  12 A , the bolt release assembly  240  may be arranged such that the left side bolt release portion  241  is a separate component from the arm  245  and the right side bolt release portion  242  is a separate component from the arm  246 . In some embodiments, the arm  245  includes a feature (e.g., feature  245 . 1 ) for engaging the left side bolt release portion  241  and/or the arm  246  includes a feature (e.g., feature  246 . 1 ) for engaging the right side bolt release portion  242 . In some embodiments, the features  245 . 1 ,  246 . 1  include a dovetail that engages a corresponding feature in the bolt release portion  241 ,  242 . In addition to a corresponding features for engaging feature  246 . 1 , the right side bolt release portion  242  may include a hole  242 . 1  such that insertion of the pin  244  secures the right side bolt release portion  242  relative to the arm  246 . In other words, insertion of the pin  244  dictates that the right side bolt release portion  242  cannot be disengaged from the arm  246 . The right side bolt release portion  242  may include a hole such that a set screw can be threaded through and at least partially into the arm  246  to secure the components together (either in addition to or in lieu of the engagement at hole  242 . 1 ). The left side bolt release portion  241  may include a similar hole  241 . 1  such that insertion of the pin  243  dictates that the left side bolt release portion  241  cannot be disengaged from the arm  245 . The left side bolt release portion  241  may include a hole such that a set screw can be threaded through and at least partially into the arm  245  to secure the components together (either in addition to or in lieu of the engagement at hole  241 . 1 ). In some embodiments, separation of the bolt release portions  241 ,  242  from the arms  245 ,  246  allows an operator to swap the external interfacing components (e.g., to change the color, texture, shape, size, and/or other characteristics of the left and/or right side bolt release portions  241 ,  242 ). 
     The left and/or right side bolt release portions  241 ,  242  may be metallic components in some embodiments. In addition, the left and right side pins  243 ,  244  may be metallic. In other embodiments, at least some portions of the left and right side bolt release portions  241 ,  242  (and/or the left and right side pins  243 ,  244 ) may be a non-metallic material (e.g., polymer). 
     As shown in  FIGS.  7 A- 8 B , the safety selector assembly  260  may interface with the safety selector hole  219   a . The safety selector assembly  260  includes at least one safety portion, and, in some cases, includes a left side safety portion  261  and a right side safety portion  262  such that the safety selector assembly  260  is fully ambidextrous. Such a configuration ensures consistent operation and ergonomics for each operator, including both right-hand dominant and left-hand dominant operators. 
     The components of any of the firearms  1  and/or the receiver assemblies  100 ,  200  described herein may be formed of materials including, but not limited to, thermoplastic, carbon composite, plastic, nylon, steel, aluminum, stainless steel, high strength aluminum alloy, other plastic or polymer materials, other metallic materials, other composite materials, or other similar materials. Moreover, the components of the firearms may be attached to one another via suitable fasteners, which include, but are not limited to, screws, bolts, rivets, welds, co-molding, injection molding, or other mechanical or chemical fasteners. 
     Different arrangements of the components depicted in the drawings or described above, as well as components and steps not shown or described are possible. Similarly, some features and sub-combinations are useful and may be employed without reference to other features and sub-combinations. Embodiments of the invention have been described for illustrative and not restrictive purposes, and alternative embodiments will become apparent to readers of this patent. Accordingly, the present invention is not limited to the embodiments described above or depicted in the drawings, and various embodiments and modifications may be made without departing from the scope of the claims below.