Patent Publication Number: US-9835398-B2

Title: Firearm trigger group module with pivoting element non-coaxial to assembly pin and method of installing a trigger group module

Description:
RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/255,768, filed Nov. 16, 2015. 
    
    
     FIELD OF THE INVENTION 
     This invention relates a so-called “drop-in” replacement trigger mechanism or module, particularly one to fit AK-pattern firearms. It provides a trigger mechanism mounted in a frame or housing independent of the firearm receiver in which at least one of the trigger and hammer does not pivot on an assembly pin extending through the receiver walls. 
     BACKGROUND OF THE INVENTION 
     AK-pattern firearms, while known for their simplicity and reliability, are not known for precision. As used herein, “AK-pattern” firearm is mean to include firearms of the Avtomat Kalashnikov or AK family, including the AK-47, AKM, AK-103, AK-74, AKS and others built on an AK type receiver, such as the Saiga family of rifles and shotguns. While simplicity of design and manufacturing are not mutually exclusive of precision, many owners of AK-pattern firearms made without precision often wish to upgrade certain features and aspects of the firearm with aftermarket products, parts, or accessories. A common “upgrade” is to install a new trigger mechanism, which generally includes a trigger member, disconnector, and hammer with associated springs and pivot pins. 
     In a standard or OEM (original equipment manufacturer) configuration, the hammer and trigger of an AK-pattern firearm are mounted to pivot on assembly pins that extend through openings in opposite side walls of the receiver. Most commonly, the receiver is formed from a flat sheet of metal that is stamped to form its finished shape. Openings in the receiver side walls that receive assembly pins may be punched or drilled in the sheet of metal prior to stamping. While it is intended that the finished location of these openings be precisely aligned with one another and provide axes of rotation transverse to the receiver and that are exactly parallel to one another, in practice, they may not be. The sometimes imprecise location of these openings makes it difficult to retrofit the firearm with a precision trigger mechanism. 
     Others have addressed this shortcoming by producing “drop-in” trigger or fire control modules that are held together as a unit by a frame or housing. However, because the final assembly pins that hold the module in place within the receiver extend through and form the axis of rotation for the trigger member and hammer, imprecision in the original receiver can result in distortion of the module causing misalignment of critical interfaces, such as the sear and/or disconnector to the hammer. Some manufacturers of these “drop-in” trigger mechanism units even expect such misalignment and instruct the installer to file or otherwise alter these critical interfaces to make the replacement trigger mechanism functional. While such custom fitting may be within the skill of a gunsmith, many end users who do their own customization by using a “drop-in” replacement trigger unit are not so skilled and expect the replacement mechanism to function properly and easily without modification of critical parts. 
     “Drop-in” replacement trigger modules designed for other types of firearms, such as the AR15, do not face these problems. The receivers of AR15-pattern firearms are generally milled with relative precision from a billet of material or a forging blank, not a stamped sheet. In addition to the relatively precise placement of openings for the assembly/pivot pins, the interior dimensions of the receiver&#39;s trigger group receiving area, including the transverse width between side walls, is typically held to close tolerances. The design of OEM parts for AR15-pattern firearms expect this level of precision, not commonly found in AK-pattern firearms, so the same can be expected from designers and manufacturers of “drop-in” replacement trigger modules for the AR15. 
     SUMMARY OF THE INVENTION 
     The present invention provides a “drop-in” replacement fire control system module, such as for a firearm that includes a portion defining a trigger group receiving area having laterally spaced apart walls with at least two pairs of laterally opposed openings for receiving assembly pins. The trigger group module comprises a module frame sized to be inserted into the trigger group receiving area and having assembly pin receiving openings positioned to align with the laterally opposed openings in the walls of the trigger group receiving area. A hammer member is pivotally supported by the module frame along a first substantially transverse axis. A trigger member is supported by the module frame along a second substantially transverse axis. At least one of the first and second substantially transverse axes is not coaxially aligned with an assembly pin receiving opening. 
     According to one aspect of the invention, the frame may include adjustment mechanisms, such as downwardly extendable set screws, to bear against the floor of the receiver or other parts mounted thereon. These adjustment mechanisms may be located longitudinally forward of the hammer pivot axis and/or rearward of the trigger pivot axis. 
     According to another aspect and method of the invention, a trigger/disconnector subassembly may be easily removed from and installed in the trigger unit&#39;s frame or housing. This allows the frame and attached hammer to be inserted into the firearm receiver without complete disassembly of the firearm followed by attachment of the trigger/disconnector subassembly after the frame has been partially or completely installed. 
     Other aspects, features, benefits, and advantages of the present invention will become apparent to a person of skill in the art from the detailed description of various embodiments with reference to the accompanying drawing figures, all of which comprise part of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Like reference numerals are used to indicate like parts throughout the various figures of the drawing, wherein: 
         FIG. 1  is a partially cut-away, isometric view of an AK-pattern receiver showing a prior art OEM trigger mechanism; 
         FIG. 2  is a isometric view of a replacement trigger unit according to one embodiment of the present invention showing the hammer in a fired or “dropped” position; 
         FIG. 3  is a forward isometric view of the unit shown in  FIG. 2 ; 
         FIG. 4  is a left-side elevation view thereof; 
         FIG. 5  is a side sectional view thereof taken generally along line  5 - 5  of  FIG. 2 ; 
         FIG. 6  is an isometric view thereof showing the hammer in a cocked or “set” position; 
         FIG. 7  is a left-side plan view thereof; 
         FIG. 8  is a partially cut-away isometric view of an AK-pattern receiver with a trigger guard and magazine release installed, but without a trigger mechanism; 
         FIG. 9  is a similar view showing a replacement trigger mechanism according to an embodiment of the present invention installed therein; 
         FIG. 10  is a partially exploded isometric view similar to  FIG. 2  in which the trigger member and disconnector unit is separated from the frame or housing; and 
         FIG. 11  is a rear sectional view taken substantially along line  11 - 11  of  FIG. 2  showing the releasable trigger member pivot pin mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawing figures, this section describes particular embodiments and their detailed construction and operation. Throughout the specification, reference to “one embodiment,” “an embodiment,” or “some embodiments” means that a particular described feature, structure, or characteristic may be included in at least one embodiment. Thus appearances of the phrases “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, and characteristics may be combined in any suitable manner in one or more embodiments. In view of the disclosure herein, those skilled in the art will recognize that the various embodiments can be practiced without one or more of the specific details or with other methods, components, materials, or the like. In some instances, well-known structures, materials, or operations are not shown or not described in detail to avoid obscuring aspects of the embodiments. 
     Referring first to  FIG. 1 , therein is shown a partially cut-away receiver  10  of typical stamped metal construction with a typical OEM hammer  12 , trigger member  14 , disconnector  16 , and safety member  18  installed therein. The hammer  12  is pivotally mounted on an assembly pin  20  that extends transversely through opposite openings  22  formed in right and left side walls  24 ,  26  of the receiver  10 . Likewise, the trigger member  14  is pivotally mounted on an assembly pin  28  that extends transversely through opposite openings  30  in the side walls  24 ,  26  of the receiver  10 . The disconnector  16  is mounted on the trigger member  14 . Likewise, a safety member  18  may be pivotally secured between opposite side walls  24 ,  26  of the receiver body  10 . Other assembly pins  32 , which also extend transversely through opposite openings in the receiver  10 , are illustrated for context, but have no significance to the present invention. All of the above described components are well known in prior art construction. 
     When a person, whether a gunsmith or an end user, wishes to replace the components of the trigger mechanism, the mounting pins  20 ,  28  are removed by driving them axially, such as with a punch, from their assembled position through the openings  22 ,  30  in the receiver  10 . Thus, the hammer  12  (and related spring) and the trigger member/disconnector  14 ,  16  (and related springs) may be removed through the open top of the receiver  10 . 
     Referring now to  FIGS. 2-7 , therein is shown at  34  a unitary fire control system or “drop-in” replacement trigger group module according to an embodiment of the present invention for an AK-pattern firearm. The unit  34  includes an open top housing or module frame  36  into which component parts of the fire control system are mounted. The unit  34  includes a hammer  38  and torsion hammer spring  40  which are mounted on a hollow sleeve  42  that extends transversely through opposite openings  44  formed in a forward portion of longitudinally extending side walls  46 ,  48  of the frame  36 . The mounting sleeve  42  has an axial interior passage with a diameter sized to receive a mounting pin  20  (not shown in  FIGS. 2-7 ) that will fit and extend through openings  22  in the receiver  10 . 
     The unit  34  also includes a trigger member  50  that is pivotally mounted on a mounting pin  52  having ends that extend through opposite openings  54  in the side walls  46 ,  48  of the frame  36 . The trigger member  50  includes a shoe or finger lever portion  56  that can be straight, curved, or formed in any desired configuration. The trigger member  50  also includes sear hook portions  58  which pivotally move with trigger member  50  in a direction opposite that of the finger lever portion  56 , above the mounting pin  52 . Also mounted on the trigger member  50  is a disconnector  60  which pivots in a limited range of motion on a pivot pin  62 , separate and independent of the pivotal movement of the trigger member  50 . A coil spring  64  biases the disconnector  60  toward a position at which its hook  66  can engage an end edge  68  of the hammer  38  when the hammer  38  is reset by cycling action of the firearm prior to release of the trigger member  50 . The trigger member  50  is biased toward a release or reset position by a trigger spring  70  (shown in  FIG. 5 ). 
     Referring specifically to  FIGS. 6 and 7 , when the hammer  38  is in a “set” or cocked position, it is held by either engagement of the sear hook portions  58  with sear flanges  72  or by engagement of the end edge  68  with the hook  66  of the disconnector  60 . In a well-known manner, engagement of the hammer  38  by the disconnector  60  is shifted to engagement by the sear surfaces  58 ,  72  as the trigger member  50  is released by the user after the cycling of the firearm action when it is fired. 
     Referring again generally to  FIGS. 2-7 , the side walls  46 ,  48  of the frame  36  include a pair of transversely aligned mounting pin openings  74 . As will be explained in greater detail later, these mounting pin openings  74  are positioned to align with the original trigger assembly pin  28  (on which an OEM trigger member  14  pivots). According to one aspect of the present invention, the trigger member  50  is mounted to pivot about a separate transverse axis spaced from and generally, but not necessarily exactly, parallel to the mounting pin opening  74 . 
     Referring to  FIG. 5 , it can be seen that the trigger member  50  includes an oversized passageway  76  which allows transverse passage of the rear mounting pin  28  while allowing pivotal movement of the trigger member  50  about the offset mounting pin  52 . In this manner, and as will be explained further below, the rear mounting pin  28  passes through openings  30  in the receiver  10  and mounting pin opening  74  in the frame  36  of the drop-in unit  34  to secure the unit  34  in place. The rear mounting pin  28  does not, however, act as a pivot axis for the trigger member  50 . Accordingly, parallel pivot axes of the hammer  38  and trigger member  50  may be maintained in precise alignment even if the mounting pins  20 ,  28  are not. 
     Referring now to  FIG. 8 , therein is shown a receiver  10  according to a typical and representative AK-pattern firearm. The receiver  10  is identical in all pertinent respects to the receiver  10  illustrated in  FIG. 1 , which shows a prior art, OEM fire control system. Various openings are provided in the floor  78  of the receiver  10 , including a forward opening  80  that receives an ammunition magazine (not shown) and, rearwardly therefrom, an opening  82  which allows a trigger to extend below the receiver  10 . A trigger guard  84  and magazine release lever  86  may be attached to the floor  78  of the receiver  10 , typically with four (4) rivets  88  at a forward position and a single rivet  90  at a rearward position. The inner surface of the floor  78  may include a transverse groove  92  aligned with the original trigger pin openings  30 , because the trigger mounting pin  28  (not shown in  FIG. 8 ) and openings  30  may be positioned extremely low in the sidewalls  24 ,  26 , at or against the floor  78  of the receiver  10 . The right sidewall  24  of the receiver  10  may include a keyhole-shaped opening for insertion of a safety member  18 . Along upper edges of the sidewalls  24 ,  26  are inwardly extending flanges  96 , which narrow the upper opening of the receiver  10  along a substantial portion of its length, including the area where the hammer and trigger mechanism of the fire control system are installed. 
     With reference to  FIG. 9 , therein is illustrated a receiver  10  partially cut away to show a modular “drop-in” fire control unit or module  34  in an installed position in a space defined between laterally spaced apart sidewalls  24 ,  26 . The module frame  36  is fully enclosed, both laterally and vertically, between the sidewalls  24 ,  26  which extend vertically at least the full height of the module frame  36 . To assemble, a forward assembly pin or hammer pin  20  is inserted through opposite openings  22  in the sidewalls  24 ,  26  of the receiver  10  and through the axial passageway of the hammer mounting sleeve  42  that is carried by and on the frame  36 . A rear assembly pin  28  (corresponding to the OEM trigger pin) is inserted through opposite openings  30  in the sidewalls  24 ,  26  of the receiver  10  and through the mounting pin opening  74  of the module frame  36 . Notably, as described above, this assembly pin  28 , which would ordinarily provide a pivot axis for a prior art trigger member, does not do so in the present invention. Instead, the trigger member  50  pivots on a separate mounting axle or pin  52 , offset from the mounting pin  28 . A pin retaining plate  98  of known form may be inserted within the receiver  10  after installation of the module  34  to engage annular grooves adjacent ends of the mounting pins  20 ,  28  to prevent axial displacement of the pins  20 ,  28 . 
     Some prior art “drop-in” replacement trigger modules for the AK-pattern platform have used adjustable set screws that bear against the floor  78  of the receiver  10  in order to prevent lateral or other unintended movement of the module after installation. In these prior examples, threaded vertical openings on lateral sides of a module frame have been positioned longitudinally in between the hammer and trigger axes of rotation, which are provided by the mounting pins  20 ,  28  in those examples. Set screws within these threaded openings are adjusted to extend below the frame of the module and bear against the floor  78  of the receiver  10 . In some cases, over-tensioning or extension of the set screws have been known to deform the floor  78  of the receiver  10 , leading to an unsightly appearance and/or other undesirable consequences. 
     According to another aspect of the present invention, adjustable securement means may be provided at forward and rearward locations on the frame  36 . These may be positioned longitudinally forward of the hammer  38  axis of rotation and/or rearward of the trigger  50  axis of rotation and assembly pin  28 . For example, as shown in  FIGS. 2-5 , internally threaded forward sockets  100 ,  102  that are laterally spaced apart may be provided at or near a forward end portion of the frame  36 . Threaded set screws  104  may be engaged in the openings  100 ,  102  to extend below a bottom edge of the frame  36  and may be adjusted from above through the openings  100 ,  102 . If desired, the threaded sockets  100 ,  102  may extend at slightly forward and/or outwardly diverging angles, as shown. Also if desired, the set screws  104  may be selected to have a sufficiently short length that a second set screw  104   a  may be threaded into the socket  100 ,  102  after the first, providing thread tension between the associated set screws to lock them against unintended rotation. Another internally threaded socket  106  may be provided at or near a rear portion of the frame  36  to receive an adjustable set screw  104 . This may be at a location longitudinally aft of the mounting pin opening  74  that receives mounting pin  28  and/or aft of the mounting pin  52  axis for the trigger member  50 . 
     As can be observed from a comparison of  FIGS. 8 and 9 , the forward sockets  100 ,  102  and set screws  104  may be positioned to align with and bear against inside heads of the rivets  88  that secure the forward end of the trigger guard  84  and magazine release  86  to the floor  78  of the receiver  10 , rather than against the material of the floor panel  78  itself. Likewise, the rear socket  106  and set screw  104  may be positioned to bear against a rivet  90  which secures the trigger guard  84  at its rear end. In this manner, the unevenness that may be created by the varied heights of the assembly rivets  88 ,  90  may be compensated for and dimpling or distortion of the floor panel  78  can be avoided. Additionally, the three points of contact provide a greater degree of exact aligning adjustment for the module  34 . 
     Referring now also to  FIGS. 10 and 11 , according to another aspect of the present invention and novel installation method, the replacement module  34  may include a trigger subassembly  108  that is separable as a unit from the frame  36 . Because the upper flanges  96  of the AK-pattern receiver  10  create a narrowed upper opening, insertion of the replacement module  34  without complete disassembly of the firearm may not be possible when the module  34  is fully assembled. When a barrel and stock (not shown) are assembled on the receiver  10 , top access can be restricted between the flanges  96  and ends of the receiver  10  are closed off. In order to allow easy insertion, the trigger subassembly  108  is separable, allowing the frame  36  and hammer  38  assembly to be inserted first, followed by installation of the trigger subassembly  108 . 
     As best depicted in  FIG. 11 , an inwardly depressible and spring-loaded mounting pin mechanism  52  may be constructed from first and second telescoping members  110 ,  112  outwardly biased by an internal spring  114  in opposite axial directions. Accordingly, the members  110 ,  112  may be squeezed together, such as with a tool, to disengage them from openings  74  in the frame  36  and allow the trigger subassembly  108  to be upwardly released and removed from the frame  36 . Reinstallation of the trigger subassembly  108  is easily accomplished by squeezing together the mounting pin members  110 ,  112 , such as with fingers or a tool, and sliding it into place along vertical inner guide channels  116  that are aligned with the trigger-mounting openings  54  in the frame  36 . 
     As previously described, the trigger member  50  of the illustrated embodiment may include an enlarged passageway  76  positioned to allow movement of the trigger member  50  when a mounting pin  28  is inserted through the mounting pin opening  74  of the frame  36  and enlarged passageway  76 . In this embodiment, the trigger sub assembly must be reinstalled on the frame  36  before insertion of the rear assembly pin  28 . If desired, however, the trigger member  50  could be formed with an open bottom slot (not shown), instead of the enlarged passageway  76 , to allow removal and reinstallation of the trigger sub assembly  108  after the frame  36  has been secured to the receiver  10  with the rear assembly pin  28 . 
     Accordingly, the present invention provides a replacement fire control system module and/or method of assembly for a firearm in which at least one of the original assembly pins that act as a pivot access for the hammer and trigger is used only to secure the module frame  36  to the receiver  10  and not as a pivot axis for moving parts of the replacement module. This construction allows the pivot axes of the hammer and trigger to be maintained precisely parallel to each other when the module is installed, even if the mounting holes for the hammer and trigger pins in the receiver are not precisely aligned. 
     While one or more embodiments of the present invention have been described in detail, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. Therefore, the foregoing is intended only to be illustrative of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be included and considered to fall within the scope of the invention, defined by the following claim or claims. Unless expressly specified, steps of a method disclosed or claimed herein may be performed in a different sequence and/or some steps may be performed simultaneously.