Abstract:
This surrogate lower receiver tool makes it possible to retract the slip ring element on a conventional M16/M4 family weapon when the hand guards must be removed or reinserted. As will be appreciated by those in the gun repair trade, in the disassembly or reassembly of such weapons the handling of the slip ring element is often a manually challenging and painstaking activity. This disclosed tool handily is an assist in such activities. The surrogate lower receiver tool makes it possible to remove and install hand guards without a lower receiver being installed on the firearm, thus avoiding the risk of damage to or loss of the lower receiver.

Description:
U.S. GOVERNMENT INTEREST 
     The inventions described herein may be made, used, or licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     BACKGROUND AND SUMMARY OF INVENTION 
     This tool makes it possible to more conveniently handle the removal and installation of hand guards (forearms) on a conventional M16/M4 family weapon when the lower receiver subassembly is not available or not desired. As will be appreciated by those in the gun repair trade, in the disassembly or reassembly of such weapons the handling of the hand guards and allied elements is a challenging, tricky and painstaking activity often requiring two individuals, and a good deal of hand gripping strength combined with pulling strength, thus greatly needing improved assist. This disclosed tool handily is a great addition and assists in such activities. 
     The description as follows includes directional designations such as up, down, left, right, lateral, transverse, longitudinal, top, bottom, vertical, and the like, that are taken from the perspective of a firearm (e.g., a conventional AR-10/AR-15/M16 family, style, platform, or pattern rifle and M4 pattern carbine, and variants thereof) as typically held and operated by a user. 
     The description assumes the level of knowledge held by an ordinary armorer, gunsmith, repair or assembly technician, user, operator, maintenance personnel, and the like for a conventional AR-15/M16 pattern rifle and M4 pattern carbine, and variants thereof, and the respective components and operation thereof. The environment forms no part of the invention. Likewise, designations such as “a”, “an”, and “the” are not to be construed to be limited to a singular item or action unless apparent from the context or definitely described as such. 
     As used herein, elements having numbers more than 9 and less than 100 generally refer to conventional elements known in the art by one having ordinary skill, while elements number 100 and above refer to the present invention, or elements, components, and the like thereof. 
     Conventional elements include:
       10 : rifle, firearm, weapon, carbine, and the like; generally AR-10, AR-15/M16 rifle, M4 carbine;     12 : upper receiver assembly;     14 : lower receiver assembly;     16 : hand guard (forearm, fore grip) group (assembly);     18 : end cap;     20 : slip ring, collar, delta ring;     22 : hand guards;     30 : front mounting lug of the upper receiver  12 ;     32 : rear mounting lug of the upper receiver  12 ;     36 : pivot pin—at front of the lower receiver  14 ;     38 : take down pin—at rear of the lower receiver  14 ;     40 : magazine well—at front of the lower receiver  14 ;     42 : bolt carrier group—internal allied components of the lower receiver  14 ;     50 : hand guard removal tool;     52 L: left arm of the hand guard removal tool  50 ;     52 R: right arm of the hand guard removal tool  50 ;     54 : end plate of the hand guard removal tool  50 ; and     56 : engagement flange of the hand guard removal tool  50 .   

     As further understood by those having ordinary skill in the art, the conventional M16/M4/AR-15 family weapons are made to an exacting technical data description that is agreed upon by manufacturers and that provides interchangeability and modularity within and between manufacturers using such designation. As such, designation of the subject firearms as being within M16/M4/AR-15 family weapons provides the indication (designation) of de facto standardization of the components so designated. Thus, such designation is distinct and particular as used in connection with claims directed thereto. 
     During assembly and repair operations of AR-15/M16 pattern (style or family) rifles and M4 pattern (style or family) carbines (i.e., weapons), and variants thereof (referred to as guns, weapons, or firearms hereafter), the hand guards (i.e., mechanism, assembly, etc.) of such weapons, e.g., forearms, fore ends, hand grips, etc., as is well known to those skilled in the art of the assembly, maintenance, and repair of the weapons are often removed and installed (or reinstalled). Such repairs may include tasks such as but not limited to installation of new hand guards to replace damaged hand guards, upgrading of obsolete hand guards, and replacement of damaged gas tubes barrels, front sights, etc. 
     Referring to  FIGS. 1 and 2 , a left side elevation view of a conventional AR-15/M16 pattern rifle and/or M4 carbine  10  is illustrated;  FIG. 2  is a broken partial view. Also referring to as well  FIGS. 3(A-C) , top, right side and rear elevation views, respectively, of a conventional hand guard removal tool  50  is illustrated. The firearm  10  comprises an upper receiver subassembly  12 , and a lower receiver subassembly  14 . 
     The upper receiver subassembly  12  comprises a hand guard group  16 , and end cap  18 , and a slip ring (e.g., delta ring, collar, etc.)  20 . The hand guard group  16  comprises a pair of hand guards  22  that are configured to matingly engage the end cap  18  at the front, the slip ring  20  at the rear, and each other longitudinally to form a substantially solid structure that is urged and held (retained) in place via a slip ring spring (not shown). The details of the hand guard group  16  and allied components are well known to those of ordinary skill in the art. Further, details may be found, for example, in U.S. Pat. Nos. 3,090,150; 4,536,982; and 4,663,875, which are incorporated by reference in their entirety. One implementation of the slip ring spring may be found, for example, in U.S. Pat. No. 3,090,150 as element  28 . 
     The upper receiver  12  further comprises a front mounting lug  30  and a rear mounting lug  32 . The upper receiver  12  matingly engages the lower receiver subassembly  14  and the subassemblies  12  and  14  are held in place as a single unit via a pivot pin  36  at lateral holes in the front of the lower receiver  14  and through a hole in the front lug  30 , and a takedown pin  38  at lateral holes in the rear of the lower receiver  14  and through a hole in the rear lug  32 . The lower receiver  14  further comprises a magazine well  40  and a bolt carrier group  42 . As is well known in the art, the magazine well  40  is configured to receive an ammunition magazine to feed cartridges into the weapon  10 . During the conventional process of removal or installation of the hand guard assembly  16 , the firearm bolt is retained within the upper receiver  12  by way of the lower receiver  14  and allied components. 
     In one example of conventional removal and/or installation of the hand guards  22 , the conventional hand guard removal tool  50  generally includes a pair of lever arms  52 L (left), and  52 R (right), an end plate  54 , and an engagement flange  56 . The end plate  54  may be generally triangularly shaped having a first end of each of the arms  52 L and  52 R fixed outwardly from the plane of the triangle in the same generally perpendicular direction at two respective vertices and the engagement flange  56  fixed at the third vertex and protruding in the same direction as the arms  52 L and  52 R. When viewed from a side as shown, for example, on  FIG. 3B , the conventional tool  50  has a generally hook (“J”) shape. 
     The end plate  54  is sized such that the arms  52 L and  52 R are spaced to loosely intersect the outer perimeter of the slip ring  20  (shown in phantom on  FIG. 3A , for reference) at about the fore-aft midpoint. The flange  56  is sized to loosely fit into the magazine well  40 . The end plate  54  is further sized and shaped such that the arms  52 L and  52 R are substantially perpendicular to the main axis of the weapon  10 . That is, the arms  52 L and  52 R are pointing away from the top side of the rifle  10  when the conventional tool  50  has the engagement flange  56  inserted into the magazine well  40  and the arms  52 L and  52 R are straddling the slip ring  20 . 
     In some alternative conventional examples (not shown), the conventional hand guard installation and removal tool  50  is implemented as a single metallic rod that is bent into a shape similar to the conventional tool  50  as illustrated on  FIGS. 3(A-C)  and described herein. In another example of convention retraction of the slip ring  20 , makeshift lever-type arrangements of paracord or rope wrapped around the slip ring  20  and threaded through the magazine well  40  of the lower receiver  14  similarly to the operation of the conventional tool  50 . The methods of use of such alternative conventional examples are essentially the same as described herein in connection with the conventional tool  50  on  FIGS. 1 ,  2 , and  3 (A-C). 
     With the conventional tool  50  mounted to (positioned on) the rifle  10  as described above, the operator squeezes the arms  52 L and  52 R towards each other (see, direction arrows, AS, on  FIG. 3A ) and substantially simultaneously forces the arms  52 L and  52 R rearward (see, direction arrow, AD, on  FIGS. 1 and 3B ) to compress the slip ring spring and slide (retract) the slip ring  20  rearward to release the hand guards  22  during removal and provide for insertion of the hand guards  22  during installation. 
     Another conventional example of a tool to aid the retraction of the slip ring  20  is shown and described in U.S. Pat. No. 6,269,577, issued to Hardy. U.S. Pat. No. 6,269,577 is directed to tool to assist in the field removal of a rifle&#39;s hand grips or hand guards, comprising a pair of releaseably joined split ring segments which are adapted to urge the slip ring of a rifle away from the rifle&#39;s mounting ring, thereby freeing the rifle&#39;s hand grips. Elastic means are provided linking the device with the rifle&#39;s stock thereby permitting pressure to continuously urge the slip ring away from the mounting ring. 
     However, while the tool of U.S. Pat. No. 6,269,577 does appear to provide a user a more firm grasp of the slip ring  20 , the tool of U.S. Pat. No. 6,269,577 does not appear to provide the leverage gained via the arms  52 L and  52 R of the conventional tool  50 . Further, the tool of U.S. Pat. No. 6,269,577 contains many small components that may be misplaced and, in any case, may cause a more time consuming operation than implementation of the conventional tool  50 . 
     In another conventional method of installation and removal of the hand guards  22 , no external tools are used. The operator, or sometimes multiple operators, manually grip and retract the slip ring  20  while simultaneously removing the hand guards  22  from the upper assembly  12 . However, many hand guards  22  are too difficult to readily remove manually, even with the aid of a second person. As such, most users (owners, gunsmiths, armorers, repair technicians, etc.) perform AR-15/M16 pattern rifle and M4 pattern carbine hand guard installation and removal via the use of magazine well  40  of the lower receiver  14  of the rifle  10  and the conventional lever device  50  to retract the slip ring  20  as described above. 
     Currently used actual lower receivers  14  have the deficiencies that they are large and heavy (especially when a butt stock is installed), and expensive, subject to theft, may be damaged during the hand guard installation and removal process, and lower receivers  14  are a regulated article that is federally registered as a firearm. Many weapon owners, armorers and gunsmiths have multiple upper receiver assemblies  12  on which repair and maintenance are performed. The damage of a lower receiver  14  may be financially burdensome, and the loss of a lower receiver  14  may result in possible criminal investigation and/or prosecution. 
     Conventional devices and methods to retain the bolt carrier group  42  during handling and/or storage of the upper receiver  12  include securing the bolt carrier group  42  via tie wraps, cord, and the like, or with a commercially available AR-15/M16 Upper Receiver Bolt Saver apparatus from Black Dog Machine, LLC, Nampa, Id. 
     Thus, there is a need and a desire for a system and a process that overcomes one or more of the deficiencies of conventional devices and processes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The description herein assumes the level of knowledge held by an ordinary armorer, gunsmith, repair or assembly technician, user, operator, maintenance personnel, and the like for a conventional AR-15/M16 pattern rifle and M4 pattern carbine, and variants thereof, and the respective components and operation thereof. The description may include directional designations such as up, down, left, right, lateral, transverse, longitudinal, top, bottom, vertical, and the like, that are taken from the perspective of a firearm (e.g., a conventional AR-10/AR-15/M16 family, style, platform, or pattern rifle and M4 pattern carbine, and variants thereof) as typically held and operated by a user. 
     The surrogate lower receiver tool of the present invention eliminates the need for an actual lower receiver  14 , thusly provides a non-regulated (i.e., not subject to government registration), compact, low cost, and light weight tool to aid the installation and removal of the hand guards of AR-15/M16 pattern rifle and M4 pattern carbine hand guards. The surrogate lower receiver tool is generally mounted on the upper receiver  12  in lieu of the lower receiver  14  to aid the removal and installation of the hand guards  22  (i.e., the subassembly  16 ). 
     The surrogate lower receiver of the present invention is implemented in connection with a conventional tool  50 , and thusly eliminates the inconvenience and expense of a second person that is recommended when a single person is unable to manually perform the hand guard installation and removal processes, as is many times the case. 
     The surrogate lower receiver of the present invention may also be implemented as at least one embodiment that further provides retention of the firearm bolt carrier group. Such a bolt carrier group retention feature may reduce or eliminate possible damage or loss of the bolt carrier group during the hand guard installation and removal processes, or during storage of the upper receiver assembly  14 . 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a left side elevation view that illustrates a conventional, prior art AR-15/M16 rifle including the upper and lower receiver sections, and a conventional hand guard tool that is used in conventional removal and installation of the hand guards; 
         FIG. 2  is a broken view of a portion of the left side the upper receiver of the firearm of  FIG. 1 ; 
         FIGS. 3(A-C)  are top, right side, and end views of the conventional hand guard tool of  FIG. 1 ; 
         FIG. 4  is an isometric view from the front of a surrogate lower receiver tool of the present invention; 
         FIG. 5  is a sectional view of the surrogate lower receiver tool of  FIG. 4 ; 
         FIG. 6  is a broken left side view that provides more detail of a hook channel component of the surrogate lower receiver tool of  FIG. 4 ; 
         FIG. 7  is a sectional view that illustrates a bolt retainer component of the surrogate lower receiver tool of  FIG. 4 ; 
         FIGS. 8(A-B)  are partial bottom elevation views that illustrate two embodiments of the hook channel component of the surrogate lower receiver tool of  FIG. 4 ; 
         FIG. 9  is a left side elevation view of the hook channel component of the surrogate lower receiver tool of  FIG. 8B , also illustrated as inset  9 A is an illustration of a spring component of the surrogate lower receiver tool of  FIG. 4 ; 
         FIG. 10  is a top elevation view of the hook channel component of the surrogate lower receiver tool of  FIG. 8A ; 
         FIGS. 11(A-D)  are rear partial, left side partial, front partial, and top elevation views of the bolt retainer component of the surrogate lower receiver tool of  FIG. 4 ; and 
         FIG. 12  is a broken view of the left side of the firearm upper receiver illustrating the mounting of the tool of  FIG. 4  during the hand guard installation and/or removal process. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 ,  2 , and  3 (A-C), as described above, illustrate the conventional, prior art AR-15/M16 rifle and/or M4 carbine  10  that includes the conventional upper receiver  12  and the conventional lower receiver  14 , and the use of a conventional hand guard removal and installation tool  50  during hand guard installation and removal. 
     The description as follows is directed to a firearm surrogate lower receiver tool  100  (wherein elements of the tool  100  are numbered 100 and above), and includes directional designations such as up, down, left (L), right (R), lateral, transverse, longitudinal, front, forward, back/backward/rearward, top, bottom, vertical, and the like, that are generally taken from the perspective of a firearm (gun, weapon, and the like, e.g., a conventional AR-10/AR-15/M16/M4 family, style, platform, or pattern rifle and carbine, and variants thereof as designated element  10  on  FIG. 1  as typically held and operated (e.g., fired). 
     The description is generally related to and made in connection with the AR-15/M16 pattern (platform, style, or family) rifles and M4 pattern carbines (i.e., weapons), and variants thereof (referred to as guns, weapons or firearms hereafter). The description is generally applicable to removal and installation of the hand guard subassembly  16  (e.g., the hand guards  22 ), and the surrogate lower receiver tool  100  may be advantageously implemented in connection with other firearms having the same or similar hand guards. The surrogate lower receiver tool is generally mounted on the upper receiver  12  in lieu of the lower receiver  14  to aid the removal and installation of the hand guard subassembly  16 . That is, the surrogate lower receiver tool generally mimics the performance of the conventional lower receiver  14 . 
     Elements (numbered 100 and above) of the present invention may include but are not necessarily included in all embodiments and are not limited to:
       100 : surrogate lower receiver (tool, apparatus, device, assembly, kit, and the like);     102 : mounting bar;     104 : hook channel;     106 : bolt retainer;     110 : retaining pins (generally two);     120 L,  120 R: left and right, respectively, front retaining holes (in side portions of mounting bar  102 );     122 L,  122 R: left and right, respectively, rear retaining holes (in side portions of mounting bar  102 );     126 L,  126 R: left and right, respectively, front axis holes (in side portions of mounting bar  102 );     128 L,  128 R: left and right, respectively, rear axis holes (in side portions of mounting bar  102 );     130 : front axis member (axle) (between the mounting bar  102  and the hook channel  104 );     132 : rear axis member (axle) (between the mounting bar  102  and the bolt retainer  106 );     136 : slot (in the mounting bar  102 );     140 : front spring;     142 : rear spring;     150 : stop edge (of the hook channel  104 );     152 : tang (tab, extension, and the like);     156 : spacers (when implemented, generally two);     158 L,  158 R: laterally opposing left and right hook axle holes; and     160 : axle retainer.   

     Referring to  FIG. 4 , an isometric view of a firearm hand guard removal and installation (or insertion) surrogate lower receiver tool (i.e., apparatus, device, unit, kit, assembly, etc.)  100  is shown from the front and left side. The tool  100  is generally implemented in connection with removal and installation of the hand guard group  16  of a conventional AR-15/M16 pattern rifle and/or M4 pattern carbine  10 , and variants thereof (i.e., gun, firearm, weapon, etc.) having an upper receiver assembly (e.g., the receiver  12 ). On  FIG. 4 , the kit  100  is illustrated as generally oriented and deployed when installed on the upper receiver  12 . For clarity of explanation, the upper receiver  12 , the hand guard removal tool  50 , and details of the spring loaded operation of the hook channel  104  and the bolt retainer  106  are not shown on  FIG. 4 . Refer also to  FIGS. 5-11  for additional illustration and description. 
     The surrogate lower receiver  100  generally comprises a mounting bar  102 , a hook channel  104 , a bolt retainer  106 , and a pair of mounting (fastening, retaining, holding, etc.) pins  110 . The mounting bar  102  is generally implemented as a channel having two (e.g., left and right) vertical walls and a horizontal floor (e.g., bottom). The open channel portion of the mounting bar  102  is generally positioned upward when the surrogate lower receiver  100  is used. At opposing (i.e., front and rear) ends the mounting bar  102  further comprises pairs of lateral retention holes through the side walls of the channel and substantially across from each other (e.g., left and right front retention holes  120 L and  120 R, respectively; and left and right rear retention holes  122 L and  122 R, respectively). The hole  122 L, a mirror of the hole  122 R, while illustrated, is not labeled. 
     The channel member  102  generally has a width selected to provide a snug fit across the width of the front mounting lug  30  and the rear mounting lug  32  of the upper receiver  12 . The longitudinal distance between the left front retention hole  120 L and the left rear retention hole  122 L, and likewise, the longitudinal distance between the right front retention hole  120 R and the right rear retention hole  122 R, are selected to be substantially the same as (e.g., match) the longitudinal distance between the lateral pivot pin mount hole in the front mounting lug  30  and the lateral take down pin hole in the rear mounting lug  32 . 
     The diameters of the lateral hole through the front mounting lug  30 , the left front retention hole  120 L, and the right front retention hole  120 R are substantially equal. The diameters of the lateral hole through the rear mounting lug  32 , the left rear retention hole  122 L, and the right rear retention hole  122 R are substantially equal. The diameter of the retention pins  110  is selected to be slightly smaller than that of the pivot pin  36  and the take down pin  38  such that the retention pins  110  may easily and quickly slide in and out of the combination of the lateral hole through the front mounting lug  30 , the left front retention hole  120 L, and the right front retention hole  120 R; and the combination of the lateral hole through the rear mounting lug  32 , the left rear retention hole  122 L, and the right rear retention hole  122 R. 
     In a preferred embodiment, the retention pins  110  may be implemented as cinch pins which are known in the art and having a loop or circular stop on one end and ball-spring detent retention on the other end. When the retention pins  110  are implemented as cinch pins, one or more lanyards may be connected between the retention pins  110  and the mounting bar  102  to prevent or reduce the likelihood of loss of the retention pins  110 . 
     In another example, the retention pins  110  may be implemented as bolts having a head and with an appropriate torque retention nut. In another example, the retention pins  110  may be implemented as unthreaded push pins having a head at one end and internal spring retention. When the retention pins  110  are implemented as unthreaded push pins, magnets (not shown) may be affixed on the outer surface of the mounting bar  102  proximate to the intended location of the heads of the retention pins  110  to provide or aid retention. In another example, the retention pins  110  may be implemented having circular grooves for retention via C clips. In another example, the retention pins  110  may be implemented having cross drilled holes for retention via hairpin clips. In other examples, the pins  110  may be implemented as push-pull, single, or double acting quick release pins. 
     The hook channel  104  is pivotally attached to the mounting bar  102  via a front axis member  130  that is laterally fastened via laterally opposing left and right front axis holes  126 L (not shown, mirror of the right front axis hole  126 R), and  126 R, respectively, in the sides of the walls of bar  102 ; and laterally opposing left and right hook axle holes  158 L (left, shown on  FIG. 6) and 158R  (right, not shown, mirror of the left hook axis hole  158 L) in the hook channel  104  to form a lateral revolute (pivot) joint. 
     The bolt retainer  106  is pivotally attached to the mounting bar  102  via a rear axis member  132  that is laterally fastened via left and right, respectively, laterally opposing rear axis holes  128 L and  128 R in the sides of the walls of bar  102  and the bolt retainer  106  to form a lateral revolute (pivot) joint. 
     The hook channel  104  is generally pivotally biased against the outer surface of the bottom of the mounting bar  102  via a front spring  140 . The bolt retainer  106  is generally pivotally biased against the inner surface of the bottom of the mounting bar  102  via a rear spring  142 . The springs  140  and  142  may rotationally force the hook channel  104  and the bolt retainer  106 , respectively, similarly to how a bail element on a mousetrap is forced by its spring. 
     The hook channel  104  is generally channel-shaped, having the open side inward spring biased against the mounting bar  102 . The hook channel  104  comprises a stop edge  150  that is oriented laterally across the hook channel  104 . When the hook channel  104  is unfolded (i.e., deployed, opened out, put into position for hand guard removal), the edge  150  generally rests against the outer surface of the bottom of the mounting bar  102  and prevents further rotational movement of the hook channel  104 . When the hook channel  104  is unfolded, the angle between the mounting bar  102  and the hook channel  104  is generally at or slightly less than perpendicular to the rear. The hook channel  104  is otherwise biased by the front spring  140  about the front axis member  130  such that the bottom of the channel  104  rests against the bottom of the mounting bar  102 , and the walls of the hook channel  104  are nested over the walls of the mounting bar  102 . 
     On  FIGS. 4 and 5 , the surrogate lower receiver  100  is shown with the hook channel  104  and the bolt retainer  106  in deployed positions as would be the case when the tool  100  is installed on the upper receiver  12  and held in place via the mounting pins  110 . However, when the surrogate lower receiver  102  is not installed on the upper receiver  12 , the hook channel  104  is generally biased against the mounting bar  102  by the front bias spring  140  (see, for example  FIG. 6  and related description). 
     The bolt retainer  106  is generally L-shaped, having a long leg of the L that is at an angle that is substantially perpendicular to the inner surface of the bottom of the mounting bar  102  when the bolt retainer  106  is deployed, and a short leg that rests against the inner surface of the floor of the mounting bar  102  and acts as a stop to prevent further (rearward) rotation of the bolt retainer  106 . The length of the long leg of the bolt retainer  106  is selected to interface within the recess in the bolt carrier group  42  (as illustrated in phantom on  FIG. 5 ) and thus retain (i.e., prevent reward movement of) the bolt carrier group  42 . 
     However, when the surrogate lower receiver  102  is not installed on the upper receiver  12 , the long leg of the L of the bolt retainer  106  is generally biased against the floor of the mounting bar  102  by the rear bias spring  142  (see, FIGS.  7  and  11 (A-D) and related descriptions). 
     Referring to  FIG. 5 , a sectional side view of the surrogate lower receiver  100  taken at the line  5 - 5  of  FIG. 4  is shown. In one embodiment, the axis member  130  may be staked, welded, or otherwise firmly fixed to the hook channel  104  such that the axis member  130  and the hook channel  104  rotate as a single member. Likewise, the axis member  132  may be staked, welded, or otherwise firmly fixed to the bolt retainer  106  such that the axis member  132  and the bolt retainer  106  rotate as a single member. 
     The front spring  140  is generally implemented having a spiral shape and a first leg on the first end and a second leg on the second end. The inner surface of the spiral front spring  140  surrounds (wraps around) the front axis member (axle)  130 . In one example, the front axis member  130  may be cross drilled to receive the first leg of the spring  140  and the second leg of the front bias spring  140  is positioned against the inner surface of the bottom of the bar  102  under tension such that the hook channel  104  is biased to and nested against the mounting bar  102  by the front bias spring  140 . 
     The rear bias spring  142  is generally implemented having a spiral shape and a first leg on the first end and a second leg on the second end. The inner surface of the spiral rear spring  142  surrounds (wraps around) the rear axis member (axle)  132 . In one example, the rear axis member  132  may be cross drilled to receive the first leg of the rear spring  142  and the second leg of the rear spring  142  is positioned against the inner surface of the bottom of the bar  102  under tension such that the long leg of the L of the retaining bolt  106  is biased against the mounting bar  102  by the rear bias spring  142 . 
     In alternative embodiments where the legs of the springs  140  and  142  are positioned against the inner surface of the mounting bar  142  to provide rotational force to the hook channel  104  and the bolt retainer  106 , the axes  130  and  132  are generally not cross drilled to receive a spring leg. In such embodiments, the axles  130  and  132  may include: (i) a bolt and nut subassembly, where the nut includes torque retention; (ii) a shaft with a head on one end and is grooved on the other end to receive a C-clip retainer; (iii) a shaft with a head on one end and on the other end is cross-drilled to receive a pin retainer (e.g., cotter pin, hairpin, and the like). 
     The fore-aft (longitudinal) position of the axle  130  (i.e., the left and right front axis holes  126 L and  126 R) is selected such that, when the hook channel  104  is deployed, the inner surface of the channel portion of the hook channel  104  is at substantially the same location as is the front of the magazine well  40  of the lower receiver  14 . 
     The fore-aft (longitudinal) position of the axle  132  (i.e., the left and right rear axis holes  128 L and  128 R) is selected such that, when the bolt retainer  106  is deployed, the front surface of the long leg of the L shaped bolt retainer  106  is at substantially the same location as is the rear edge of a notch (recess) the bolt in the bolt carrier group  42  (shown in phantom) of the lower receiver  14 . As such, the bolt retainer provides a definite stop that may prevent rearward, longitudinal movement of the bolt carrier group  42 . 
     Referring to  FIG. 6 , a partial left side view of the surrogate lower receiver  100  is shown. The hook channel  104  is illustrated in the closed (e.g., retracted down, folded, stowed, etc.) position as is the case when the hook channel  104  is biased against the mounting bar  102  by the front bias spring  140 . 
     Referring to  FIG. 7 , a sectional side view of the surrogate lower receiver  100  taken at the line  7 - 7  of  FIG. 4  is shown. However, contrary to the illustration of  FIG. 4 , the bolt retainer  106  is illustrated in the closed (e.g., retracted down, folded, stowed, etc.) position as is the case when the retaining bolt  106  is biased against the mounting bar  102  by the rear bias spring  142 . 
     Referring to  FIGS. 8(A-B) , partial bottom views of the surrogate lower receiver  100  illustrating two alternative embodiments of the hook channel  104  are shown. In the embodiment shown on  FIG. 8A , the hook channel  104  further comprises a pair of spacers  156 . The spacers  156  are generally tubular (i.e., cylindrical with a through hole that receives the shaft  130 ). Leg portions of the hook channel  104  that receive the axis member  130  are substantially parallel to the main body of the hook channel  104  and are positioned away from the mounting bar  102  via the spacers  156  on the left and right sides of the axis member  130 . In the embodiment shown on  FIG. 8B , the leg portions of the hook channel  104  that receive the axis member  130  are bent inward to rest against the mounting bar  102  and receive the axis member  130 . The spring  140  has a leg that extends through a slot  136  in the mounting bar  102  and is bent to urge the hook channel  104  against the mounting bar  102 . 
     Referring to  FIG. 9 , including sectional view  9 A taken at line  9 A- 9 A on  FIG. 8B , a left side view of the hook channel  104  is shown. Referring back to  FIG. 8B , as well as  FIG. 9 , in another example embodiment, the lateral stop edge  150  may further include a laterally oriented tang (e.g., tab, extension, protrusion, etc.)  152 . The tang  152  may extend from the bottom of the hook channel  104  and may provide strength and stability to the interface of the hook channel  104  and the mounting bar  102  when the hook channel is opened for use during hand guard removal or installation. Sectional view  9 A more clearly illustrates a leg of the spring  140  at rest on the inner surface of the bottom portion of the channel bar  102 . 
     Referring to  FIG. 10 , a top view of the embodiment of the hook channel  104  as illustrated on  FIG. 8A , is shown. The channel body of the hook channel  104  generally has a width, HW, and a length, HL, that are sized to loosely but securely receive the engagement flange  56  (illustrated for reference in phantom) to provide firm purchase (i.e., solid resting position) during use of the surrogate lower receiver  100  (as described below in connection with  FIG. 12 ). 
     Referring to  FIGS. 11(A-D) , back (rear) partial, left side partial, front partial, and top views, respectively, of the bolt retainer  106  are shown. For clarity, the axle  132  is only illustrated on  FIG. 11B . As previously described, the bolt retainer  106  is generally L-shaped, having the long leg of the L that is spring biased via the rear spring  142  against the mounting bar  102  when the surrogate lower receiver  102  is not in use to retain the bolt carrier group  42 , and a short leg that rests against the mounting bar  102  when the surrogate lower receiver  102  is implemented to retain the bolt carrier group  42 . The bolt retainer  106  further comprises an axle retainer  160  that is formed at the right angle of the L. In another example, the axle retainer  160  may be implemented as a tubular element. The axis member (axle)  132  is generally laterally positioned through the axle retainer  160  and the walls of the mounting bar  102 . 
     Gaps between the axle retainer  160  and the body of the bolt retainer  106  are sized to accommodate the legs of the rear spring  142 . 
     When the bolt retainer  106  is unfolded (i.e., deployed, opened out, put into position for hand guard removal), the long leg of the L generally rests against the rear surface the bolt component near the firing pin within a recess of the bolt carrier group  42  (see,  FIG. 5 ). However, when the bolt carrier group  42  is not present in the upper receiver  12 , the bolt retainer  106  is left in the undeployed position. When the bolt retainer  106  is unfolded, the angle between the long leg of the L and the inner surface of the bottom of the mounting bar  102  is generally at or slightly less than perpendicular. The bolt retainer  106  is otherwise biased by the front spring  142  about the rear axis member  132  such that the long leg of the L rests against the bottom of the mounting bar  102 . 
     As is known to one of skill in the art, a number of appropriate arrangements may be implemented to bias the hook channel  104  against the mounting bar  102  and the bolt retainer  106  against the mounting bar  102 ; i.e., provide rotational tension (torque) about an axis between two elements on an axis, as well as the embodiments described. The legs (i.e., arms, extensions, prongs, etc.) of the bias springs  140  and  142  are generally positioned such that the hook channel  104  and the bolt retainer  106  are biased (i.e., urged, forced, pushed, actuated, etc.) against the mounting bar  102 , that is, to closed (retracted) positions to provide a compact, easy to store device. 
     The mounting bar  102 , the hook channel  104 , and the bolt retainer  106  may be made from metal (e.g., steel, aluminum, or the like), filled plastic (e.g., nylon, polymer), high strength polymer, composites, or any other material having the requisite materials properties as would be known to one of skill in the art. The pins  110  and the springs  140  and  142  are generally made of steel or steel alloy. 
     Referring to  FIG. 12 , a partial left side view illustrating use of the surrogate lower receiver tool  100  is illustrated in connection with the conventional upper receiver  12  and the conventional hand guard tool  50  is shown. To mount the surrogate lower receiver  100  to the upper receiver  12  when the bolt carrier group  42  is installed, the bolt retainer  106  is rotated to the vertical position. This step is not implemented when the bolt carrier group  42  is not present. The surrogate lower receiver  100  is aligned at the lugs  30  and  32  with the hook channel  104  at the front, at the front, one of the pins  110  is generally inserted laterally through a hole  120 L or  120 R, and through the hole in the lug  30  and across the mounting bar  102 . Similarly, at the rear one of the pins  110  is generally inserted laterally through the hole  122 L or  122 R, and through the hole in the lug  32  and across the mounting bar  102 . The order of insertion of the pins  110  can be either front-rear or rear-front. As such, the surrogate lower receiver  100  is matingly engaged to the upper receiver  12 . 
     The hook channel  104  is then deployed to a position substantially perpendicular to the main axis of the upper receiver  12 , the hand guard tool  50  is positioned with the legs  52  straddling the slip ring  20 , and the engagement flange  56  is inserted into the channel of the hook channel  104 . As is done when a conventional lower receiver  14  is installed on the upper receiver  12 , the legs  52  of the hand guard tool  50  are squeezed together and pushed rearward to retract the slip ring  20 . When removal or installation of the hand guard group  16  is complete, the pins  110  are pulled free and the surrogate lower receiver  100  is removed from the upper receiver  12 . However, when desired, the surrogate lower receiver  100  may be left installed on the upper receiver  12  such that the upper receiver may be stored, handled, shipped, and the like with the bolt carrier group  42  securely held in place via the bolt retainer  106  of the surrogate lower receiver  100 . 
     A number of simplified alternative embodiments of the tool  100  may be implemented. For example, at the joints between the bar  102  and the hook channel  104 , the spring  140  may be deleted and the rotational position of the hook channel  104  may be maintained via friction, ribs, dimples, surface roughness, striations, and the like at the interface between the bar  102  and the hook channel  104 . Similarly, the spring  142  may be deleted and the bolt retainer  106  may be held in position via frictional fit of the bolt retainer  106  to the mounting bar  102 . 
     In another embodiment, when the need for retention of the bolt carrier group  42  is not anticipated by the user, the bolt retainer  106  and the spring  142  may be deleted. 
     The firearm tool  100  is generally used by a user such as an armorer, gunsmith, repair or assembly technician, maintenance personnel, and the like. However, a home (non-commercial) user may find the surrogate lower receiver tool  100  to be a useful aid as well. The surrogate lower receiver  100  generally mimics a lower receiver  14  and, in connection with the conventional hand guard removal tool  50 , provides an improved system and method of retracting the slip ring  20 . 
     While the invention may have been described with reference to certain embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.