Abstract:
A lower receiver for an AR15 type firearm is provided comprising a non-detachable magazine movable in the receiver between a first locked position where the firearm is ready for use (i.e. cocked and ready for firing) and a second non-operational position where the magazine is lowered sufficiently to open a window or aperture in the receiver enabling the user to load fresh shell cartridges into the magazine. A locking mechanism also is provided for controlling the position of the non-detachable magazine between the first and second positions. A child-proof lock further is provided to prevent undesired activation of the locking mechanism. Additionally, a novel locking plate disposed between the lower receiver&#39;s frame and the receiver&#39;s stock extension tube provides enhanced support for the extension tube and an accessory attachment point for a shoulder strap or sling.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application bases a claim of priority for all purposes on my prior provisional application Ser. No. 61/999,791, filed Nov. 5, 2014, which provisional application hereby is incorporated herein and made part of this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention relates generally to firearms and more specifically to improvements in the lower receiver of an AR-15 type firearm which has been modified so as to meet legal restrictions for the civilian use of such firearms. 
         [0004]    Description of the Prior Art 
         [0005]    The present invention relates generally to firearms of the well-known AR-15 type, and more particularly to AR15-type firearms that are modified to meet the many Federal and State rules and regulations restricting the availability and use of such equipment. In my prior patent, U.S. Pat. No. 6,739,082, entitled “Firearm With Fixed Cartridge Magazine Top,” there is fully described an AR15-type firearm suitably configured to meet restrictive gun laws by, among other things, providing a lower receiver portion without a bottom opening for a magazine clip, and further providing a top-inserted magazine sized to receive no more than ten (10) cartridges. The entire disclosure of my prior &#39;082 patent hereby is incorporated into this application and made part hereof by this reference. The firearm disclosed in my &#39;082 patent, designated the “California FAB10,” met with considerable commercial success because it included a detachable magazine and was able to feature the popular “pistol grip” while still complying with the restrictive gun laws on the books at that time. More recent changes to restrictive firearm laws have made it untenable to employ civilian AR-15 type firearms such as the prior art “FAB10” with both a detachable magazine and the enormously popular “pistol grip” feature. Additionally, in order to reload the prior art “FAB10,” it was and is necessary to depress the “take down” pin and pivot the upper receiver open much like a double-barrel shot gun, a procedure most users found to be cumbersome and undesirable. 
       SUMMARY OF THE INVENTION 
       [0006]    To overcome the forgoing and other disadvantages, the present invention, briefly summarized, comprises a lower receiver for an AR-15 type firearm, modified to meet legal restrictions, and characterized by a non-detachable magazine movable in the receiver between a first locked position where the firearm is ready for use (i.e. cocked and ready for firing) and a second non-operational position where the magazine is lowered sufficiently to open a window or aperture in the receiver enabling the user to load fresh shell cartridges into the magazine. The lower receiver further includes a locking mechanism for controlling the position of the non-detachable magazine between first and second positions by means of a depressible actuating button on one side of the receiver and a movable lock finger on the other side of the receiver. The lock finger is adapted to engage a recess in the magazine to lock it into the first position. Upon depression of the actuating button, the lock finger exits the recess releasing the non-detachable magazine for downward movement in the receiver. A child-proof lock further is provided to prevent the magazine lock finger from operating. Additionally, a novel locking plate disposed between the lower receiver&#39;s frame and the receiver&#39;s stock extension tube provides enhanced support for the extension tube and an accessory attachment point for a shoulder strap or sling. 
         [0007]    The above brief description sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be better understood, and in order that the present contributions to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will be for the subject matter of the appended claims. 
         [0008]    It is therefore a primary object of the present invention to provide a new and improved lower receiver apparatus for a firearm which has all of the advantages of the prior art and none of the disadvantages. 
         [0009]    The forgoing and still other objects of the invention, along with the various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention will be better understood and the above objects as well as objects other than those set forth above will become more apparent after a study of the following detailed description thereof. Such description makes reference to the annexed drawings wherein: 
           [0011]      FIG. 1  is a side view in elevation showing a preferred embodiment of the firearm lower receiver apparatus of the invention installed in a complete firearm, between a firing unit, a stock and a pistol grip. 
           [0012]      FIG. 2  is an enlarged elevational side view of the lower receiver apparatus shown in  FIG. 1 . 
           [0013]      FIG. 3  is a top plan view taken along line  3 - 3  of  FIG. 2 . 
           [0014]      FIG. 4  is an enlarged fragmentary side view in elevation of a portion of the lower receiver apparatus designated by circle  4  in  FIG. 2  and showing a first unlocked lock position of the magazine lock assembly according to the invention. 
           [0015]      FIG. 5  is an enlarged fragmentary side view in elevation of a portion of the lower receiver apparatus designated by circle  4  in  FIG. 2  and showing a second locked position of the magazine lock assembly according to the invention. 
           [0016]      FIG. 6A  is an enlarged elevational view of the lower receiver apparatus of  FIG. 1  showing the opposed other side of the receiver apparatus with the non-detachable magazine portion in a first fire-ready position. 
           [0017]      FIG. 6B  is an enlarged elevational view of the lower receiver apparatus of  FIG. 1  showing the opposed other side of the receiver apparatus with the non-detachable magazine portion in a second non-fire-ready operational position and with the magazine loading window in an open position. 
           [0018]      FIG. 6C  is an enlarged elevational view of the lower receiver apparatus of  FIG. 1  showing the opposed other side of the receiver apparatus with the non-detachable magazine portion in a third or intermediate non-fire-ready operational position. 
           [0019]      FIG. 7  is a cross-sectional view taken along line  7 - 7  in  FIG. 6A  showing the magazine lock in an engaged locking position. 
           [0020]      FIG. 8  is the same cross-sectional view taken along line  7 - 7  in  FIG. 6A , but showing the magazine lock actuating button in a depressed condition such that the magazine is in an unlocked condition adapted for movement within the receiver apparatus. 
           [0021]      FIG. 9  is an enlarged elevational side view of the lower receiver apparatus of  FIG. 1  showing the magazine locked in an intermediate position within the receiver apparatus. 
           [0022]      FIG. 10  is a cross-sectional view taken along line  10 - 10  of  FIG. 9 . 
           [0023]      FIG. 11  is an elevational side view of the magazine portion of the lower receiver apparatus removed from the magazine and showing the two vertically spaced locking apertures and the stop lug thereon. 
           [0024]      FIG. 12  is an exploded assembly view of the apparatus showing the novel locking plate according to the invention disposed between the stock extension tube and the receiver apparatus. 
           [0025]      FIG. 13  is a front side view in elevation of the locking plate of  FIG. 12  taken along line  13 - 13  of  FIG. 12 . 
           [0026]      FIG. 14  is a rear side view in elevation of the locking plate of  FIG. 12  taken along line  14 - 14  of  FIG. 12 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    With reference to the drawings, a new and improved firearm lower receiver with non-detachable magazine apparatus embodying the principles and concepts of the present invention will be described. 
         [0028]    Turning initially to  FIG. 1 , the improved firearm apparatus of the present invention generally is designated by reference sign  10  and comprises a barrel  12 , a frame comprising an upper receiver  14  and a lower receiver  16 , a stock  18 , a pistol grip  20 , and a firing mechanism. The upper receiver and the firing mechanism are well known in the art. The barrel  12  is fixedly attached to the upper receiver  14  as is also well known in the art and the upper receiver is pivotally attached to the lower receiver  16  at a hinge pin  22 . The firing mechanism of the firearm includes a trigger and hammer assembly in the lower receiver  16 , and a bolt carrier and bolt assembly longitudinally slidably mounted in the upper receiver  14 . The upper receiver  14  also includes a takedown pin attached to the lower receiver  16  for fixedly attaching the upper receiver to the lower receiver. The firing mechanism and the takedown pin are not shown because these parts are well known in the art and outside the scope of the present invention. 
         [0029]    Turning to  FIGS. 2-11 , in accordance with an important feature of the invention, the lower receiver  16  includes a non-detachable magazine assembly  24  mounted for limited slidable longitudinal movement in a magazine reception well  26  provided in a portion of the lower receiver&#39;s frame generally indicated by reference sign  28 , which frame portion  28  thus provides a housing for the magazine assembly  24 . The frame portion  28  has a first or left side  29  as viewed in  FIGS. 2 and 9 , and an opposed second or right side  31  as viewed in  FIGS. 6A, 6B and 6C . As in the prior &#39;082 patent, magazine assembly  24  suitably is sized and configured to retain and dispense no more than 10 cartridges. Additionally, magazine assembly  24  can include interiorly thereof the same or similar cartridge magazine spring and cartridge magazine follower arrangement shown in FIG. 3 of my &#39;082 patent. 
         [0030]    The magazine reception well  26  extends completely and longitudinally through the lower receiver frame portion  28  and is characterized by a generally quadrilateral transverse cross-sectional shape to define a passage for the magazine assembly  24  telescopically received therein. More specifically, the magazine reception well  26  is defined by a pair of opposed spaced apart side walls  30 ,  32  and a pair of opposed spaced apart end walls  34 ,  36 , substantially as depicted. The walls extend longitudinally between a common top edge  38  and a common bottom edge  40 . A concave channel  42  is provided in top edge  38  and the interior surface of side wall  30  extends longitudinally from the top or upper edge  38  of the magazine reception well toward the bottom edge  40 . However, the channel  42  does not intercept the bottom edge; rather the channel terminates a distance above the bottom edge  40  of the magazine reception well  26  to define a lateral stop ledge or shoulder  43  recessed into the interior surface of side wall  30 , substantially as shown in  FIG. 10 . 
         [0031]    As mentioned, and as in my prior &#39;082 patent, non-detachable magazine assembly  24  is hollow and suitably sized and configured to receive no more than ten cartridges of the type normally used in AR-15 style firearms. In a departure from the prior art, the magazine assembly  24  of the present invention is adapted to be nested telescopically inside the magazine reception well  26  and can be indexed longitudinally between first, second and third positions without pivoting open the upper receiver from the lower receiver, as will be further explained below. To facilitate such arrangement and action, magazine assembly  24  comprises a front wall  44 , an opposed rear wall  46 , a first side wall  45 , and an opposed second side wall  47 . A somewhat enlarged bottom panel  48  closes off and seals the bottom of the magazine. A tamper-resistant, hardened, non-removable security screw  50  is installed in magazine side wall  47  for preventing withdrawal of the magazine from the reception well in an upward direction. The security screw  50  is emplaced after the magazine is installed in the reception well to ensure that the magazine is and remains non-detachable from the lower receiver frame  28 . The security screw  50  is such that it may only be removed by special cutting tools requiring mutilation and damage to the magazine. 
         [0032]    The top of the magazine assembly is defined by an annular edge  52  and a pair of opposed, upright, slightly-bent-inwardly clips  54 ,  56  for receiving and holding a cartridge in place by snap-fitment therebetween as is well known in the art. A Delrin® cartridge follower platform  58  closes off the interior of the magazine and when the magazine is empty of cartridges, the follower platform  58  is resiliently urged against the underside of clips  54 ,  56  by a spring assembly (not shown) located interiorly of the magazine assembly between the bottom of the follower platform  58  and the floor defined by the bottom panel  48 . Essentially, the top portion of the magazine assembly  24 , the Delrin® follower platform  58  and the resilient spring are the same as or similar to that shown in FIG. 3 of my prior &#39;082 patent and fully described therein. 
         [0033]    It will be observed, in accordance with this invention, when the firearm is in the first or fire-ready position, the magazine assembly  24  has sufficient length so that the bottom portion thereof including bottom panel  48  extends below bottom edge  40  of the magazine reception well  26  of frame portion  28 , substantially as depicted in  FIGS. 1, 2 and 6A . 
         [0034]    As best seen in  FIGS. 10 and 11 , the magazine assembly  24  includes on or in wall  44 , a first locking aperture  58 , a second locking aperture  60 , and a protruding stop lug  62  with the first and second apertures  58 ,  60  and the stop lug  62  being located and spaced apart in longitudinal axial alignment substantially as depicted. It will be noted that the location of the imaginary longitudinal axis of the apertures  58 ,  60  and stop lug  62  coincides with the imaginary central axis of concave channel  42  when the magazine assembly  24  is disposed within the magazine reception well ( FIGS. 1-3 and 6-11 ). 
         [0035]    Turning to  FIGS. 6A, 7 and 8 , a lock-release assembly generally indicated by reference sign  64  is provided in the lower receiver  16  above trigger guard  66  and comprises a release button  68 , a push-rod  70 , a transverse extension member  72 , and a locking finger  74 . More specifically, a first transverse recess  76  having a bore of first size communicates with a second passage  78  of second size, which latter, in turn opens into a third transverse oblong or quadrilaterally-shaped recess  80  of third size. As shown in the drawings, the push-button  68  is disposed in the first recess  76 ; the push-rod  70  is disposed in the second passage  78 ; and the transverse extension member  72  and locking finger  74  are disposed in the third recess  80 , respectively. 
         [0036]    A spiral spring  82  surrounding the push-rod is seated in recess  76  substantially as depicted and functions to normally resiliently bias the lock release assembly  64  in the position shown in  FIG. 7 . In that position, the locking finger  74  is adapted to lockingly engage either first locking aperture  58  or the second locking aperture  60  in the wall  44  of magazine assembly  24  depending upon the relative longitudinal position of the magazine assembly  24  within the magazine reception well  26 . 
         [0037]    When the locking finger  74  lockingly engages the first locking aperture  58  in wall  44  of magazine assembly  24  ( FIG. 7 ), the magazine assembly is resiliently-locked in the position shown in  FIG. 6A . This is the first or fire-ready position of the firearm. Release of such locking engagement may be effected in accordance with the present invention by depressing button  68  in the direction of arrow  84  ( FIG. 8 ). This action causes the button  68  to move in first recess  76  against the resilience of spiral spring  82 , causing the push-rod to move laterally in second passage  78 , which in turn causes the extension member  72  and locking finger  74  thereon to move laterally out of third recess  80  thereby withdrawing the locking finger  74  from the first locking aperture  58  in magazine assembly wall  46  and freeing the magazine for longitudinal movement within the magazine reception well ( FIG. 8 ). 
         [0038]    Another important feature of the present invention is the ability to load or re-load cartridges into the magazine assembly  24  even though the firearm is in the closed condition, i.e. the upper receiver is locked to the lower receiver by the takedown pin. Access to the top portion of the magazine assembly when the firearm is in such “closed” condition is provided by a suitably sized and configured magazine access window or aperture  86  located in the wall of the frame portion on one side thereof proximal to lock-release push-button  68  and trigger guard  66 , substantially as depicted in  FIGS. 6-8 . In order to use access window  86  to load cartridges into magazine  24 , the magazine must be in its third position as shown in  FIG. 6B . To index the magazine  24  from say the first position of  FIG. 6A  to the third position of  FIG. 6B , the push-button  68  is depressed in the direction of arrow  84  ( FIG. 8 ), freeing the magazine  24  to fall via gravity toward the bottom of the reception well  26 . The magazine  24  will come to rest when the protruding stop lug  62  engages the lateral stop ledge or shoulder  43  recessed into the interior surface of side wall  30 . When this action occurs, the longitudinal travel of the magazine downward in the reception well  26  is arrested and magazine  24  remains via gravity in the third position of  FIG. 6B . Access window  86  thus permits individual cartridges to be loaded into the top of the magazine  24  because in the third position of  FIG. 6B  the top portion of the magazine  24  is reposed slightly under the lower rim of the window  86 . When the magazine  24  is re-loaded as desired, the magazine  24  ultimately can be slid upwardly in the reception well  26  until the locking finger  74  resiliently engages the first locking aperture  58  in magazine assembly wall  44  under the influence of spiral spring  82  in recess  76  and the magazine is returned to the first or “fire ready” position of  FIG. 6A . Of course, it will be appreciated that when the magazine is longitudinally moved upward from the third position of  FIG. 6B  to the first position of  FIG. 6A , the rear wall  46  of the magazine closes off and seals the access window  86 . 
         [0039]    In accordance with the invention, the magazine assembly  24  optionally is adapted to be parked in a second position substantially as shown in  FIGS. 6C, 9 and 10 . In this second position, the pair of opposed, upright, slightly-bent-inwardly cartridge-holding clips  54 ,  56  are located slightly below the common top edge  38  of the magazine reception well  26 , and the locking finger  74  joined to transverse extension member  72  is adapted to lockingly engage the second or top locking aperture  60  in the wall  44  of magazine assembly  24  ( FIG. 10 ). An index mark  88  preferably is provided suitably located on magazine assembly  24  more or less as depicted in  FIGS. 9 and 11  to assist the user in moving magazine  24  to the second position, i.e. by aligning the index mark  88  with the common bottom edge  40  of the magazine reception well ( FIG. 9 ). When this occurs, the locking finger  74  will be opposite the second locking aperture  60  such that release of the push-button  68  will cause the locking finger to enter the second locking aperture  60  and lock the magazine into its second position. It will be appreciated that by suitable operation of the locking-release push button  68 , the slidable magazine assembly  24  may be moved within frame  24  “from” any of the first, second or third position of the magazine “to” any of the other remaining positions of the magazine  24 . In the second position of the magazine  24 , the firearm may be cycled without a cartridge in the bolt chamber and this condition is preferred particularly during storage of the apparatus. 
         [0040]    Another important feature of the invention is the provision of a “childproof” safety lock assembly for preventing use of the firearm apparatus  10  by youngsters or the like. This feature is shown in  FIGS. 4 and 5  and comprises a pivotal or rotatable locking disc  90  fixed to an axle pin  92  suitably captured for rotation in the frame  24 , and a transversely protruding resilient detent  94  also suitably captured in frame  24 . Locking disc  90  further includes a flat surface  96  and first and second peripheral notches  98 ,  100  substantially as depicted. 
         [0041]    In the non-locking condition, disc  90  has its flat surface substantially parallel and in alignment with the bottom edge of third transverse oblong or quadrilaterally-shaped recess  80 . With this state of affairs depicted in  FIG. 4 , disc  90  is locked into a static “non-locking” position by the engagement of detent  94  in the second notch  100  and the disc  90  therefore has no effect on the transverse movement of transverse extension member  72  and locking finger  74  in and out of recess  80  (i.e. see  FIGS. 7 and 8 ). By pressing resilient detent  94  inward (into the plane of the paper as viewed in  FIG. 4 ), disc  90  can be rotated clockwise via rotatable axle pin  92  to the “locking position” as viewed in  FIG. 5  and held in a static “locking” position by the engagement of resilient detent  94  and the first notch  98 . In that “locking” condition ( FIG. 5 ), the disc effectively blocks movement of extension member  72  and locking finger  74  in and out of recess  80  ( FIGS. 7 and 8 ). As a result, the lock-release assembly (push-button button  68 ) is dis-enabled, effectively locking the magazine in either the first or second position depending upon whether the locking finger  74  is engaged with first locking aperture  58  or second locking aperture  60  in wall  44  of magazine assembly  24 . 
         [0042]    Stated otherwise, operation of the childproof locking assembly to the “locked” condition shown in  FIG. 5 , is effective to “lock” the magazine within the reception well  26  in either the first position ( FIG. 2 ) or in the second position ( FIG. 9 ). The reason the locking assembly of  FIGS. 4 and 5  is “childproof” is that it is relatively difficult to depress resilient detent  94  and cause the disc to rotate between its locking and non-locking positions. A sharp pointed instrument and manual dexterity are required to execute this action and most children and even some adults will have substantial difficulty doing so. It will be appreciated that to return the childproof safety lock to the “unlocked” condition of  FIG. 4 , the foregoing action is reversed, i.e. the detent  94  is depressed into the body of frame  24  preferably using a sharp-pointed device, and disc  90  is rotated counterclockwise (as viewed in  FIGS. 4 and 5 ) about the imaginary rotation axis defined by pin  92  until flat surface  96  once again is in substantially parallel alignment with the bottom edge of third oblong recess  80 , and the resilient detent  94  is lockingly engaged in second peripheral notch  100 . 
         [0043]    In summary, the present invention provides an AR-15 style firearm with a pistol grip feature and a non-detachable magazine obviating pivoting open the upper receiver from the lower receiver in order to reload cartridges into the non-detachable magazine. While it is still possible to use the takedown pin to pivot open the firearm, and remove the bolt assembly, the magazine still cannot be withdrawn through the top of the magazine reception well because of the aforementioned security screw  50 , or through the bottom of the magazine reception well because of the action of the protruding stop lug  62  on the magazine engaging the lateral stop ledge or shoulder  43  recessed into the interior surface of side wall  30  of the magazine reception well. 
         [0044]    Turning to  FIGS. 2 and 12-14 , still yet another important feature of the present invention is the provision of a novel, multi-purpose locking plate generally indicated by reference sign  102 . Locking plate  102  functions to securely lock the receiver extension tube  104  to the extension tube collar  106  located conventionally on the rear portion of the lower receiver frame  24 . The extension tube  104  (partially depicted) which houses the firearm&#39;s recoil spring and serves as an anchor for an attachable stock, and the collar  106  are well known in the art. In this regard, collar  106  includes a female-threaded bore  108  whereas extension tube  104  terminates in a complimentary, male-threaded plug portion  110 . A conventional castle nut  112  also is provided having a female-threaded bore  114  suitably configured and sized to threadedly and complimentarily engage the male-threaded plug portion  110  of extension tube  104 . As is well known in the art, the extension tube plug portion  110  is intended to be screw-threaded into engagement with the receiver collar via threaded bore  108 , and the castle nut  112  threadedly engaged on the plug portion  110  is then tightened down against the collar  106  to securely lock these parts in place. 
         [0045]    In accordance with the invention, and as substantially depicted in  FIG. 12 , novel locking plate  102  is interposed between the collar  106  and castle nut  112  for the general purpose of providing a more secure locking connection between these parts, and to achieve other advantages as will become more evident from the ensuing description. 
         [0046]    As viewed in  FIGS. 12-14 , locking plate  102  preferably is of unitary one-piece construction and comprises an upper ring portion  114  and an integral lower enlarged portion  116 . Upper ring portion  114 , in turn, has an inner diameter suitably sized to permit the ring (and therefore the plate  102 ) to be slidingly received over the tube extension male-threaded plug portion  110 . A tang  118  extends radially upward from the bottom of the ring portion  114  for ultimate mating engagement in a longitudinally extending keyway or slot  120  suitably provided in the outer surface of the extension tube plug portion  110 . 
         [0047]    The lower or bottom portion  116  of locking plate  102  is transversely enlarged as best depicted in  FIG. 12  to define a curved (concave) supporting surface or seat  122  suitably sized and configured to engage the exterior or outer confronting surface of castle nut  112  when the latter is tightened down against the upper ring portion  114  and collar  106  ( FIG. 2 ). To help maintain the upright orientation of the locking plate shown in  FIGS. 12-14  when the parts are assembled ( FIG. 2 ), the left side of the locking plate includes a protrusion or button  124  adapted to be received in a complimentary sized and configured recess  126  suitably formed in the rear surface  128 ′ of the receiver frame  24  ( FIG. 12 ). 
         [0048]    Locking plate  102  further comprises a laterally extending integral tab  128  having an opening  130  for serving as an attachment point for a shoulder strap or sling. As indicated by the dashed outline in  FIG. 13 , the tab  128  and its defined opening  130  may optionally extend to the right or to the left depending upon the personal dictates of the user. 
         [0049]    Additionally, as substantially depicted in  FIGS. 13 and 14 , locking plate  102  may be provided with a first female-threaded recess  132  extending vertically through lower or bottom portion  116  in substantial alignment with tang  118  and intercepting curved supporting surface or seat  122 . A first male-threaded set screw  134  is threadedly engaged within first recess  132  and may be tightened against the castle nut  112  engaging supporting surface or seat  122  after the parts are assembled together. By this action, the locking plate  102  provides enhanced support for and secure locking engagement of the tightened-down castle nut  112 . 
         [0050]    Locking plate  102  preferably includes a second female-threaded recess  136  extending transversely through lower or bottom portion  116  for receiving a second male-threaded set screw  138 . Second recess  136  aligns with a resilient detent  140  suitably seated in the frame  28  and which may be used conveniently for accessing the takedown pin when it is desired to pivot open the upper receiver from the lower receiver as is well known in the art. 
         [0051]    In summary, the novel locking plate  102  of the present invention by providing a tang  118  adapted to engage keyway  120 , and a button  124  adapted to be received in recess  126  of the rear surface  128  of the receiver frame  24 , ensures that the receiver extension tube is maintained in proper alignment when the castle nut  112  is tightened, and provides enhanced support for the extension tube and the receiver&#39;s stock. Additionally, the locking plate  102  includes a first set screw  134  for tightening against the castle nut  112 ; provides a concave surface or seat  122  for providing enhanced support of the castle nut in its tightened condition; provides a second set screw for permitting access to the receiver&#39;s takedown pin without removing the castle nut and locking plate; and provides a convenient attachment point for a sling or strap. 
         [0052]    The manner of usage and operation of the instant invention is believed apparent from the above disclosure, and accordingly, no further discussion relative to the manner of usage and operation need be provided. 
         [0053]    The foregoing detailed description is considered as illustrative only of the principles of the invention. Numerous modifications and changes will readily occur to those skilled in the art and therefore, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents falling within the broad scope of the subject matter described above may be resorted to in carrying out the present invention. 
         [0054]    Hence, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification.