Patent Publication Number: US-11650023-B2

Title: Selective fire firearm systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/275,253 filed Sep. 23, 2016 and entitled “SELECTIVE FIRE FIREARM SYSTEMS AND METHODS” which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     One or more of the embodiments relate generally to firearms, and more particularly, for example, to a firearm configured for fully automatic open bolt or semi-automatic closed bolt firing. 
     BACKGROUND 
     Semi-automatic and fully automatic firearms are well known. Semi-automatic firearms shoot one bullet each time that the trigger is pulled. Fully automatic firearms continue shooting as long as the trigger is pulled and they have not exhausted their ammunition and are typically capable of relatively high rates of fire, e.g., cyclic rates. For example, the M16 and the M4 have a nominal cyclic rate of 700 to 950 rounds per minute. 
     Because fully automatic firearms are capable of such high cyclic rates, they are prone to a variety of problems. For example, sustained fully automatic fire may result in barrel overheating. Barrel overheating is particularly problematic when high capacity magazines, such as SureFire&#39;s 60 round and 100 round magazines, are being used. High capacity magazines allow longer periods of sustained fire since fewer magazine changes are required to fire a given number of rounds. Fewer magazine changes provide less time for the barrel to cool. Thus, the barrel, as well as other parts of the firearm, may be subjected to increased heat. 
     Often, the ability to keep firing is limited by barrel overheating, which may result in malfunction of the firearm. For example, cartridges chambered into an overheated barrel may detonate prematurely, e.g., cook off, particularly in closed bolt firearms. 
     If care is not taken, a fully automatic firearm, particularly a fully automatic mode of a selective fire firearm with a closed bolt semi-automatic mode, can cease fire in a closed bolt position with a cartridge chambered in the hot barrel. A fully automatic firearm resting in closed bolt position may be hazardous as cook off may occur. 
     The possibility of a cartridge firing due to cook off may have disastrous consequences in battlefield and police situations. The involuntary detonation of ammunition has resulted in injury or loss of life in such instances. Therefore, it would be desirable to provide systems and methods for facilitating improved firearm function. 
     BRIEF SUMMARY 
     In accordance with embodiments further described herein, features are provided that may be advantageously used in one or more firearm designs. More specifically, in accordance with an embodiment, a firearm is provided that fires fully automatic from the open bolt position to prevent cook off and have greater controllability, and fires semi-auto from a closed bolt position for accuracy. Furthermore, in accordance with an embodiment, a simple and reliable selector mechanism is provided for selection of mode of operation (e.g., semi-auto closed bolt or fully automatic open bolt) of the firearm. According to an embodiment, a selective fire firearm having a fully automatic mode and a semi-automatic mode may be provided with a trigger group having a trigger prop that, in the automatic mode, functions to ensure that, when the trigger is released following fully automatic fire, the firearm will cease fire in an open bolt position. In this way, the risk of cook off due to a cartridge chambered in a hot barrel after fully automatic fire may be reduced or eliminated. 
     For controllable full auto we use a variation of the Davis Recoilless Gun Principal that shot a bullet out the front and a sandbag out the rear so all the recoil was in the weight of the sandbag, none in the gun, no matter what the gun weighed. Instead of the sandbag we use the weight of the bolt carrier group and buffer and instead of throwing them out the back, we gave them enough room and spring force to gradually stop them before they hit a rear wall. Our gun is therefore not recoilless, but it delivers the lowest possible recoil force stretched out over the longest possible time (the time between one shot and the next) and it doesn&#39;t matter what the gun weighs. It does however require that each cycle be the same and should deliver half the recoil impulse while the cycling weight is decelerating rearward and half as the spring is accelerating the weight forward. If the second half of the first cycle is omitted from first shot of a full auto burst (as do most Assault Rifles), the first shot of the burst has up to twice the recoil impulse of subsequent shots which reduces full auto controllability of that burst. 
     Since changing the selector from closed bolt semi-auto to open bolt full auto will not cock the cycling weights rearward to open bolt position, the trigger mechanism provides two options. The user may use the cocking handle to cock the weights rearward or save time by immediately firing. It will then just fire the first shot of that burst from closed bolt, but cease fire in the open bolt and fire all subsequent bursts from open bolt. 
     According to an embodiment, a firearm is provided that includes a bolt carrier and a trigger group including a trigger having a trigger sear; a hammer having a notch configured to be engaged by the trigger sear to prevent firing when the trigger is released in a semi-automatic closed bolt mode of operation for the firearm; and a trigger prop configured to prevent engagement of the notch by the trigger sear in the fully automatic open bolt mode of operation. An open bolt sear may be provided and configured to block motion of the bolt carrier to prevent firing when the trigger is released in a fully automatic open bolt mode of operation. 
     According to another embodiment, a dual use trigger is provided that includes a trigger sear configured to engage a hammer notch of a standard hammer and a light pull sear configured to engage a hammer post of a light pull hammer. 
     According to another embodiment, a firearm is provided that includes a charging handle; a trigger; and a lever arm, in which, based on a position of the charging handle and a mode of operation of the firearm, the lever arm is configured to selectively block or allow a pull of the trigger. 
     According to another embodiment, an open bolt sear assembly for a firearm is provided that includes an open bolt sear, which includes a main body and at least two legs coupled to the main body, and a sear arm configured to interact with a trigger of the firearm. The main body may have a mating face that engages a bolt carrier. Furthermore, the two legs may each have engagement surfaces that engage the wall of the firearm. The open bolt sear assembly may also include a first and second pivot point that the sear arm and open bolt sear may rotate about, respectively. The open bolt sear assembly may be used in the firearm during, for example, AUTO OB mode of operation. For example, the open bolt sear is configured to block motion of a bolt carrier to prevent firing when the trigger of the firearm is released in a fully automatic open bolt mode of operation for the firearm; and the open bolt sear is configured to allow motion of the bolt carrier when the trigger is pulled in the fully automatic mode of operation. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings. The scope of the disclosure is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments, as well as a realization of additional advantages thereof, will be afforded to those skilled in the art by a consideration of the following detailed description of one or more embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are left and right side views, respectively, of a selective fire firearm having a magazine attached thereto in accordance with one or more embodiments of the present disclosure. 
         FIG.  2 A  is a cross-sectional side view of a lower receiver assembly of the firearm of  FIG.  1    in fully automatic open bolt mode according to an embodiment. 
         FIG.  2 B  is a cross-sectional side view of a lower receiver assembly of the firearm of  FIG.  1    implemented without a trigger prop in fully automatic open bolt mode according to an embodiment. 
         FIG.  3    is a cross-sectional side view of the lower receiver assembly of the firearm of  FIG.  1    in safe mode according to an embodiment. 
         FIG.  4    is a cross-sectional side view of the lower receiver assembly of the firearm of  FIG.  1    in semi-automatic closed bolt mode according to an embodiment. 
         FIG.  5    is an exploded perspective view of a trigger prop assembly of the firearm of  FIG.  1    according to an embodiment. 
         FIGS.  6 A and  6 B  are assembled views of the trigger prop assembly of the firearm of  FIG.  1    according to an embodiment. 
         FIG.  7 A  is a side view of a hair trigger assembly that includes a dual use trigger according to an embodiment. 
         FIG.  7 B  is a side view of a standard pull trigger assembly that includes a dual use trigger according to an embodiment. 
         FIG.  8 A  shows a perspective view of the standard pull trigger assembly of  FIG.  7 B  according to an embodiment. 
         FIG.  8 B  shows a perspective view of the hair trigger assembly of  FIG.  7 A  according to an embodiment. 
         FIG.  9    is an exploded front perspective view of a risk button assembly for a selective fire firearm according to an embodiment. 
         FIG.  10    is an exploded rear perspective view of the risk button assembly of  FIG.  9    according to an embodiment. 
         FIG.  11    is a perspective view of portion of a firearm having a risk button assembly according to an embodiment. 
         FIG.  12    is an elevational exploded side view of a risk button assembly according to an embodiment. 
         FIG.  13    is a perspective view of a limited motion disk of the risk button assembly of  FIG.  10    according to an embodiment. 
         FIG.  14    is a perspective view of a risk button plunger of the risk button assembly of  FIG.  10    according to an embodiment. 
         FIG.  15    is a face-on view of a selector showing various positions of the selector according to an embodiment. 
         FIG.  16    is a face-on view of the risk button assembly of  FIG.  10    in a fully automatic open bolt mode position according to an embodiment. 
         FIG.  17    is a face-on view of the risk button assembly of  FIG.  10    in a safe mode position according to an embodiment. 
         FIG.  18    is a face-on view of the risk button assembly of  FIG.  10    in a semi-automatic open bolt mode position according to an embodiment. 
         FIG.  19    is a face-on view of the risk button assembly of  FIG.  10    in a safe mode position according to an embodiment. 
         FIG.  20    is a side view of a firearm trigger showing various positions of the trigger according to an embodiment. 
         FIG.  21    is a perspective cutaway view of a portion of a firearm having a lever arm according to an embodiment. 
         FIGS.  22  and  23    are top cutaway views of a portion of a firearm having a lever arm and a charging handle with the charging handle in respective forward and rearward positions according to an embodiment. 
         FIG.  24    is a perspective view of a forward assist button of a firearm according to an embodiment. 
         FIG.  25    is a perspective view of a forward assist cover on a forward assist button of a firearm according to an embodiment. 
         FIG.  26    is a side view of a forward assist cover on a forward assist button of a firearm according to an embodiment. 
         FIG.  27    is a perspective view of a forward assist cover for a firearm according to an embodiment. 
         FIGS.  28 A and  28 B  are side views of an open bolt sear engaged and disengaged, respectively, with a bolt carrier group of a firearm according to an embodiment. 
         FIGS.  29 A- 29 D  are various views of the open bolt sear according to an embodiment. 
     
    
    
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
     An improved firearm, in accordance with one or more embodiments, has various different features that enhance the operation and use thereof. For example, a trigger group of a firearm may contain a trigger prop that props the rear of the trigger during automatic bursts such that, when the trigger is released, the propped trigger raises an open bolt sear to cease firing, but cannot engage a hammer of the trigger group (e.g., the propped trigger may be prevented from making contact with a hammer notch on the hammer). In this way, in a fully automatic mode, it can be assured that the bolt carrier group rests at the rear of the firearm (e.g., in the open bolt position) during a cease fire. By preventing the bolt carrier group from moving forward to the closed bolt position and chambering a cartridge into the heated barrel after a burst of fully automatic fire, possible cook off of cartridges may be prevented. 
     The trigger block (also referred to herein as a “trigger group”) may be configured to keep the open bolt sear raised when the firearm is changed from a fully automatic mode to a safe mode (e.g., by rotating a selector from a fully automatic position to a safe position) so that the bolt carrier group remains in the open bolt position when the firearm is in safe mode. In the safe mode, the trigger prop may be retracted from under the rear end of the trigger so that the trigger sear rises up and engages the hammer notch on the hammer and the automatic sear may be retracted thereby allowing the hammer to rest on the trigger sear. 
     The trigger block may be configured such that, when the firearm is changed from the safe mode to a semi-automatic mode, the open bolt sear may be released, so that the bolt carrier group moves forward to the closed bolt position feeding a cartridge form the magazine into the chamber without firing it until the trigger is again pulled. 
     The selector may be arranged such that the safe mode position is located between the fully automatic mode position and the semi-automatic position. Because the bolt carrier moves forward when the selector is moved from the fully automatic mode position through the safe mode position to the semi-automatic mode position, a mechanism on the selector may be provided to further prevent cook off when changing to semi-automatic mode. For example, changing from the safe mode to the semi-automatic mode may require an extra step such as depressing a button on the selector, so as to increase the user&#39;s awareness of the risk of cook off, thereby increasing the likelihood that the user will consider whether such a change is appropriate. 
     The firearm upper receiver may have a forward assist which is used to force the bolt carrier assembly forward if it gets stopped out of battery. This forward assist button can be depressed, which forces a plunger forward that engages notches cut on the bolt carrier and forces the bolt carrier forward. When the firearm is in fully automatic open bolt operation, if the trigger is pulled and the bolt carrier assembly gets stopped before firing a round and the operator uses the forward assist to force the bolt carrier forward once the bolt carrier assemble reaches its locked position the round will fire whether the trigger is still pulled or not. Once the round fires, the bolt carrier will recoil rearward with high energy and force the depressed forward assist plunger rearward into the hand of the operator causing possibly injury. 
     A cover for the forward assist such as a snap-on plastic cover may be provided that prevents an operator from pushing the forward assist button. A snap-on cover of this type may be installed on the forward assist during fully automatic open bolt operation of the firearm to help prevent injury. The snap-on cover may be configured to be optionally used and easily installed and removed in just seconds. 
     According to an embodiment, the firearm may be compatible with large capacity magazines. For example, the firearm may be compatible with 60 and 100 round magazines. The firearm may be configured to withstand the heat associated with sustained fully automatic fire. The ability to ensure that the bolt carrier group rests in a rearward, open bolt position, following a burst of automatic fire is one aspect of how the firearm may withstand the heat associated with sustained fully automatic fire. 
     One type of firearm is discussed herein as an example, though the present disclosure may be applied to other known firearms. The firearm may be made in any desired caliber. For example, each type of firearm may be made in 5.56×45 mm NATO or 6.8×43 mm. Both 5.56×45 mm NATO and 6.8×43 mm may share components. For example, both 5.56×45 mm NATO and 6.8×43 mm may generally share all components except the barrel, bolt, and magazine for a given type of firearm. 
       FIG.  1    shows a firearm  100 , according to an embodiment. The firearm  100  is capable of fully automatic (full auto) and semi-automatic (semi-auto) fire as selected by the user (e.g., firearm  100  may be a selective fire firearm). As shown, firearm  100  may include a selection mechanism  121  (e.g., a selector) for selecting between the full auto and semi-auto modes and a safe mode in which firing of the firearm is prevented. 
     Firearm  100  may be configured to fire from an open bolt position during automatic fire and from a closed bolt position during semi-automatic fire. In this way, the accuracy of the firearm may be increased in the semi-automatic mode and the risk of cook off may be reduced in the full auto mode. 
     The firearm may include an upper receiver group  103  that consists of barrel  105 , a gas system, a sight assembly, a handguard, and an optional grip  106 . The lower receiver  102  of firearm  100  may consist of the trigger group, grip  107 , and magazine well  108 . Firearm  100  may have a stock  114 , which may contain the receiver extension for the recoil buffer, main (e.g., buffer) spring, and bolt carrier group. The firearm  100  may have a magazine, e.g., magazine  101 , attached thereto. Magazine  101  may be, for example, a 60-round or 100-round magazine such as those sold by SureFire, LLC of Fountain Valley, Calif. 
     One or more embodiments provide a magazine fed, gas operated auto cycling firearm which operates generally as follows. Like all breech loading repeaters, the firearm must perform eight ammunition handling functions between one shot and the next. The firearm may feed, chamber, lock, fire, unlock, extract, eject the ammunition cartridge and cock the gun ready for the next cycle. The bolt group is involved in all eight of these functions. As a main spring drives the bolt group forward, the bolt group completes the feed by pushing the top cartridge forward out of the magazine and tilting the forward bullet end up a feed ramp and into the barrel chamber. The bolt head is rotated using a cam to lock the bolt head and the cartridge into the barrel, then, successively, firing the cartridge and completing the forward moving half of the bolt cycle. 
     As the bullet moves through the barrel, the bullet passes a gas port hole drilled in the barrel wall through which high pressure gas enters the cylinder and drives the piston rearward (or if the bolt carrier contains the cylinder, the gas acts directly on the bolt carrier), thus throwing the bolt carrier rearward and compressing the main spring. During the bolt carrier&#39;s first rearward motion a helical cam in the carrier rotates the bolt to unlock the bolt head from the barrel and then pulls the bolt rearward for the rest of the combined rearward cycle. An extractor claw on the bolt head pulls the fired cartridge case from the barrel chamber and an ejector strikes or pushes on the cartridge base opposite the extractor, pivoting the cartridge around the extractor and out through an eject port in the gun structure. The combined bolt head and bolt carrier&#39;s continued rearward motion uncovers the new top cartridge in the magazine which feeds it upward at least partially into the bolt head&#39;s return path while the rearward moving carrier and bolt cocks the spring loaded firing hammer and moves beyond (rearward of) a bolt stop, which is lifted up by the magazine follower after the last cartridge has fed from the magazine and which catches and holds the bolt and carrier group rearward so that the empty magazine may then be removed and replaced with a full one ready to resume fire without hand cocking the gun. A cocking handle, or a charging handle, may be provided in case of a misfire or other cycle malfunction. 
       FIG.  2 A  is a cross-sectional side view of a portion of the receiver of firearm  100 . The lower receiver assembly  102  includes the trigger group of firearm  100 . The trigger group may include trigger  202  with trigger sear  204 , a trigger spring (not shown), hammer  206  having a hammer notch  208 , a hammer spring (not shown), selector mechanism  210 , automatic sear  212 , trigger prop  214 , an automatic sear spring (not shown), disconnect  216 , and disconnect spring  218 . During automatic fire, the trigger works in conjunction with open bolt sear arm  220 , open bolt sear  222 , and other individual components as discussed herein. 
     The trigger group may be a drop-in trigger assembly, according to an embodiment. The trigger block assembly may be assembled outside of the firearm  100 . Once assembled, the trigger block assembly may be dropped into place in lower receiver  102 . 
     The receiver assembly also houses the bolt carrier  224  and bolt  225 , charging handle  228  as well as other components found in contemporary assault rifle receivers, such as, for example, an ArmaLite® rifle (AR) receiver. In one embodiment, a charging handle  228  may be slidably disposed in the upper receiver of the rifle to facilitate cocking of firearm  100  by pulling bolt carrier  224  rearward. 
     In an embodiment of the present disclosure, rotating the selector to the AUTO OB position does not cock the cycling weights (e.g., bolt carrier group and/or buffer) rearward to open bolt position; therefore, the user may either use the charging handle  228  to move the weights rearward or immediately fire such that the first shot is fired from the closed bolt position. For example, until a first shot is fired in a fully-automatic open bolt (AUTO OB) mode (e.g., after operating the firearm in the semi-auto mode), the first shot may be fired from a closed bolt position. On the first trigger pull for firing a full auto burst, the trigger prop spring may cause foot  226  of trigger prop  214  to swing forward under the rear portion  227  of trigger  202 . Foot  226  then props the rear of trigger  202  up at a position that prevents trigger sear  204  from engaging/holding hammer notch  208 /hammer  206  while still allowing automatic sear  212  to be raised. Therefore, hammer  206  may be held by automatic sear  212  and bolt carrier  224  may be held by open bolt sear  222  in an open bolt position when trigger  202  is released at the end of the full auto burst. Thus, the first burst of fire may begin from the closed bolt position and stop in the open bolt position, and all subsequent bursts of fire start and end in the open bolt position. A method of operating the firearm while in the fully automatic open bolt mode of operation may therefore include firing a first fully automatic burst from a closed bolt configuration; stopping the first fully automatic burst in an open bolt configuration; and firing a subsequent fully automatic burst from the open bolt configuration. 
     For example, for shots fired from the open bolt position, trigger  202  may be pulled back by the user resulting in the raising of open bolt sear arm  220 , which lowers open bolt sear  222 . This lowering of the open bolt sear  222  allows bolt carrier  224  to be released and move forward due to the buffer and buffer spring. The forward moving bolt chambers a live round from the magazine and the breech is locked by the bolt while automatic sear  212  is tripped by the back of bolt carrier  224 . When automatic sear  212  is tripped, hammer  206  is released by tripped auto sear  212  and strikes the firing pin inside bolt  225  and thus a round is fired, lugs of the barrel and bolt  225  unlock, and bolt carrier  224  travels rearward riding over hammer  206  and reengaging a top hammer notch with auto sear  212 . 
     The cycle is repeated as the main spring pushes bolt carrier  224  forward again unless trigger  202  is released to raise open bolt sear  222  to catch bolt carrier  224  or until magazine  101  is empty. If trigger  202  is released mid-fire, open bolt sear  222  is allowed to rise and hold bolt carrier  224  to the rear of the firearm  100  while the trigger prop foot  226  prevents the trigger from being released to a position in which the trigger or the disconnect catches hammer  206 . In this way, it can be ensured that the last chambered round in full auto mode is always fired before the bolt carrier is caught by the open bolt sear. As understood by one skilled in the art, the extraction and ejection of the case are the same as done by contemporary AR platforms. 
       FIG.  2 B  is a cross-sectional side view of a portion of the receiver of firearm  100  showing how automatic sear  212  and lower receiver assembly  102  may be provided without a trigger prop in one embodiment. 
     In an embodiment of the present disclosure,  FIG.  3    shows a cross-sectional view of the receiver assembly of firearm  100  having a trigger prop when selector mechanism  210  (e.g., an interior structure of selector  121  of  FIG.  1   ) has been moved from an AUTO OB position to a safe mode or SAFE position. As shown, selector mechanism  210  may be configured such that, when the selector is moved from the AUTO OB position to the SAFE position, the selector first moves the trigger prop foot  226  rearward from under the rear portion  227  of trigger  202  which allows the trigger sear  204  to rise up and engage the hammer notch  208 . The selector mechanism  210  then retracts the automatic sear (auto sear)  212  which holds the hammer  206  during full-auto fire and allows the hammer  206  to rest on the trigger sear  204 . 
     Once the selector is in the SAFE position, the bolt carrier  224  is still in an open bolt position and the selector mechanism  210  can be moved either back to open bolt full-auto position or to a closed bolt semi-auto (SEMI CB) position. If the selector mechanism  210  is rotated to the SEMI CB position it releases the open bolt sear  222  so the bolt assembly moves forward feeding a cartridge from the magazine into the chamber without firing it until the trigger  202  is again pulled firing semi-auto closed bolt. 
       FIG.  4    illustrates firearm  100  in SEMI CB mode. In semi-auto fire mode, firearm  100  may operate the same as a conventional AR firing semi-auto from a closed bolt. For example, when selector mechanism  210  is moved from AUTO OB through SAFE to SEMI CB (semi-automatic closed bolt), selector mechanism  210  disengages trigger prop  214 , automatic sear  212 , and open bolt sear  222 , and allows disconnect  216  to operate as would be understood by one skilled in the art. For example, disconnect  216  engages the underside of hammer  206  after each round is fired, and may release hammer  206  once trigger  202  is released and trigger sear  204  engages hammer notch  208 .  FIG.  4    shows bolt  225  when in battery and the lugs are locked to provide a closed breech and loaded chamber. As shown in  FIG.  4   , hammer  206  is engaged by trigger  202 . When trigger  202  is pulled, hammer  206  is released and may travel through the opening in the bolt carrier to strike the back of the firing pin, thus firing a round. The bolt carrier may then travel rearward, compressing main spring  402  and pushing buffer  404 . The force of spring  402  and buffer  404  may then push the bolt carrier group forward again, chambering a new round and resting in closed bolt position. As shown in  FIG.  4   , in SEMI CB mode, open bolt sear  222  is held down and does not affect the motion of bolt carrier  224 . Bolt catch  400  may hold bolt  225  after the final round is fired. 
     The hammer may be an aluminum hammer having a steel face. The hammer may be all steel. The hammer may be comprised of aluminum, titanium, steel, or any combination thereof. The hammer may be made of any desired material. The hammer may be hard anodized where the bolt carrier slides against the hammer. The hammer may be hardened or treated as desired where the bolt carrier slides against the hammer or on any on any other part or surface thereof. 
       FIG.  5    shows an exploded view of a trigger prop assembly  600  including trigger prop  214  and auto sear  212  for firearm  100  according to an embodiment. As shown in  FIG.  5   , trigger prop assembly  600  may include trigger prop  214  and auto sear assembly  501 . As shown, auto sear assembly  501  may include auto sear  212 , axel  500 , and pin  502 . As shown in  FIG.  5   , trigger prop  214  may have a trigger prop spring  606  that biases trigger prop  214  into various positions based on the position of selector  121  as described herein. Auto sear  212  may include an auto sear spring  604  that biases auto sear  212  into various positions based on the position of selector  121  (a selector mechanism  210 ) as described herein. 
       FIG.  6 A  shows a perspective view of trigger prop assembly  600  in an assembled configuration. As shown in  FIG.  6 A , when trigger prop assembly  600  is assembled, axel  500  may extend through openings in auto sear  212 , trigger prop  214 , auto sear spring  604 , and trigger prop spring  606  so that auto sear  212  and trigger prop  214  can be independently or jointly biased about axel  500 . As shown, trigger prop foot  226  may extend beyond an extended leg  602  of auto sear  212 . In this way, trigger prop assembly  600  may be configured so that, when selector  121  is moved from an AUTO OB position to a SAFE position, the trigger prop foot  226  can be moved rearward from under the trigger and then the automatic sear  212  can be retracted. 
       FIG.  6 B  shows a top view of trigger prop assembly  600  showing how pin  502  may extend from opposing sides of trigger prop assembly  600  (e.g., for mounting of assembly  600  within the trigger block of firearm  100 ). It can be seen in the example of  FIG.  6 B  that a portion  610  of trigger prop  214  that wraps around axle  500  may separate the trigger prop spring  606  from the auto sear spring  604 . 
     In accordance with one or more embodiments of the present disclosure, a trigger for a firearm such as firearm  100  may be provided that can be used as a standard trigger or a light pull trigger (sometimes referred to as a hair trigger or a Marksmen® trigger) by changing the hammer used in conjunction with the trigger.  FIG.  7 A  shows an example of a dual use trigger configured for use with multiple hammers in accordance with an embodiment. 
     As shown in  FIG.  7 A , a dual use trigger such as trigger  702  may include both a trigger sear  708  (standard pull) and light pull sear  710 . In the example of  FIG.  7 A , trigger  702  is implemented with a light pull hammer  700  to form a light pull trigger assembly. Light pull hammer  700  may have hammer post  704  that engages with a light pull sear  710  of dual use trigger  702 . Hammer post  704  of light pull hammer  700  may be located further away from the hammer pivot  722  than the location of a standard hammer notch, thereby increasing the moment arm of trigger  702  and reducing the amount of force on surface  726  required to release hammer  700 , thus, creating a lighter trigger pull. 
     Light pull hammer  700  may include a cutaway portion  706  that provides clearance  720  with trigger sear  708  at the location where a standard hammer notch would be located to prevent trigger sear  708  from engaging hammer  700 . In various embodiments, trigger  702  may be implemented in a firearm having a trigger prop that props rear portion  728  of trigger  702  in a full auto mode or may be implemented in any other suitable firearm. Light pull hammer  700  may include an extended portion  730  having a notch  732  configured to engage an auto sear in some modes of operation as would be understood by one skilled in the art. Hammer post  704  may be configured to engage light pull sear  710  of trigger  702  on a first side of the hammer post and to engage, in some modes of operation, a disconnect on a second opposing side  734  of the hammer post. 
     As shown in  FIG.  7 B , dual user trigger  702  may be alternatively implemented with a standard hammer  740  having a hammer notch  744  that engages trigger sear  708  at a location relatively closer to its hammer pivot  742  than light pull sear  710 . Standard hammer  740  may include an extended portion  746  having a notch  748  for engaging an auto sear in some modes of operation and may include a portion  750  for engaging a disconnect in some modes of operation. 
       FIG.  8 A  is a perspective view of the assembly shown in  FIG.  7 B  showing how portion  750  of standard hammer  740  does not engage light pull sear  710  of trigger  702  thereby allowing trigger sear  708  and hammer notch  744  to control the release of hammer  740  when trigger  702  is pulled.  FIG.  8 B  is a perspective view of the assembly of  FIG.  8 A  showing how hammer post  704  of hammer  700  extends beneath light pull sear  710  of trigger  702  to engage light pull sear  710  and control the release of hammer  700 . 
     Hammer  740  may be exchanged for hammer  700  as desired by the user to allow for a lighter trigger pull. Because trigger  702  is configured to operate with both hammers  700  and  740 , a firearm including trigger  702  may be converted from a standard pull weight to a light pull weight by replacing only the hammer and no other parts may require alteration and/or removal to change the hammer (unlike if the trigger was to be changed) and, thus, the pull weight may be changed with, for example, a standard cartridge as the tool. 
     As discussed herein, according to an embodiment, a selector mechanism can be used to select between modes of operation for a firearm such as firearm  100  (e.g., to select between an AUTO OB mode, a SAFE mode, and a SEMI CB mode). The selector may be a button, slider, rotary, switch, and/or other mechanism, as would be understood by one skilled in the art. 
     The selector mechanism can be configured such that changing the selection between the AUTO OB mode, SAFE mode, and/or SEMI CB mode merely involves moving a selector lever. However, in some embodiments, the selector mechanism can be configured such that changing the selection from open bolt (e.g., AUTO OB) to closed bolt (e.g., SEMI CB) requires an extra step. For example, changing the selection from open bolt to closed bolt can require that a button be depressed. The button can be part of the selector switch or can be separate therefrom. For example, the button can be in the middle of the selector switch. 
     Requiring that an extra step be performed in order to change from open bolt operation to closed bolt operation helps to assure that proper consideration is given regarding the propriety of this change. As those skilled in the art will appreciate, changing from open bolt operation to closed bolt operation can result in a dangerous cook off if a round is chambered while the chamber is hot. For example, a cook off can occur if a round is chambered before the chamber has cooled adequately after sustained rapid firing of the firearm. Cook offs are not likely to occur during open bolt operation since the cartridge is fired as soon as it is chambered. Thus, this extra step when changing from open bolt operation to closed bolt operation is a desirable safety feature. The extra step can cause a user to more carefully consider whether or not the chamber has had adequate time to cool. 
       FIG.  9    shows a front exploded perspective view of a selector that requires an activator, such as a button push, to change from AUTO OB mode to SEMI CB mode. The exemplary activator illustrated in  FIG.  9    is a risk button assembly that may include a fastener  902 , an ambidextrous selector  904 , a limited motion disk  906 , a risk button plunger  908 , a selector member  910  and a receiver boss  912 . The risk button assembly may also include a bias spring (not shown).  FIG.  10    shows a rear exploded perspective view of fastener  902 , ambidextrous selector  904 , limited motion disk  906 , risk button plunger  908 , selector member  910  and receiver boss  912  having lobes  914 .  FIG.  11    is a perspective view of a receiver  102  implemented with a selector  121  having an assembled risk button assembly and showing how risk button plunger  908  may be disposed centrally within ambidextrous selector  904 . Knob  1100  on ambidextrous selector may be pushed or pulled by a user to switch between AUTO OB, SAFE and SEMI CB operating modes for the firearm. However, the risk button assembly may be configured such that, to move selector  121  from the SAFE mode position shown in  FIG.  11    to SEMI CB mode, risk button plunger  908  must be pushed (e.g., using a user&#39;s finger, a bullet, or a magnet) if the selector was previously at or near the AUTO OB position. The risk button assembly may be configured such that the risk button does not stop the operator from going back and forth between either firing mode or SAFE nor does it prevent the operator from going from semi-auto to full auto, but only causes the operator to decide whether to risk a semi-auto cook off. 
       FIG.  12    is an elevational exploded view of the risk button assembly showing additional portions of selector mechanism  1202  of which selector member  910  forms a part and that extend into receiver  102  to arrange the trigger, disconnect, auto sear, open bolt sear, and hammer for various modes of operation as described herein.  FIG.  12    shows a risk button spring  1220  that may be disposed between selector mechanism  1202  and plunger  908  to bias plunger  908  outward and to allow compression of plunger  908  to move to a closed bolt mode of operation. 
       FIG.  13    is a perspective view of limited motion disk  906 . As shown in  FIG.  13   , limited motion disk  906  may be symmetric and may contain a set of limited rotation tracks  1300  and limited rotation blocks  1302 .  FIG.  14    is a perspective view of risk button plunger  908 . As shown in  FIG.  14   , risk button plunger may include two tabs  1400  that extend outward from a central button portion and guide on the limited motion disk  906 . Each tab  1400  may include a ramp  1401  on one side and a perpendicular face  1402  (sometimes referred to as a “block”) on an opposing side. When the selector is rotated from SEMI CB towards SAFE, the ramps  1401  on the risk button plunger  908  engage the limited rotation blocks  1302  on the limited motion disk  906  and start depressing the risk button plunger  908 . When the selector is in the SAFE position, the selector can be moved back to SEMI CB without a push of plunger  908 . However, if the selector is moved further towards AUTO OB it will pass the limited rotation blocks  1302  on the limited motion disk  906  and risk button plunger  908  will have to be depressed to return to the SEMI CB position. 
     When the selector is in the AUTO OB position and rotated to the SAFE position, the blocks  1402  on the risk button plunger  908  engage the limited rotation blocks  1302  and start to rotate the limited motion disk  906  until it reaches the SAFE position. From the SAFE position, if the operator attempts to continue to rotate the selector towards the SEMI CB position, the limited rotation tracks  1300  on the limited motion disk  906  will engage the lobes  914  on the receiver boss  912  and will prevent any further rotation of the selector until the risk button plunger  908  is depressed to disengage the blocks  1402  on the risk button plunger  908  from the limited rotation blocks  1302  on the limited motion disk  906 . 
     In this way, the ramps  1401  and blocks  1402  may allow the selector to rotate in one direction but block it from rotating in the other, while the limited rotation tracks  1300  allow the selector to be moved into SAFE position from either firing mode and back to the firing mode from which it came without requiring a depression of plunger  908 . 
     In some embodiments, the assembly can have a torsion spring (not shown) to bias the limited motion disk in a clockwise direction (e.g., as indicated by arrow  913  of  FIG.  10   ) to ease operation of the risk button so that when the risk button plunger is depressed, the selector does not need to be rotated at the same time. 
       FIG.  15    shows a selector  121  having a first position  1500  associated with a fully automatic open bolt (AUTO OB) mode, a second position  1502  associated with a safe (SAFE) mode, and a third position  1504  associated with a semi-automatic closed bolt (SEMI CB) mode of operation. The risk button assembly may be configured to prevent a turn of the selector from the first position  1500  through the second position  1502  to the third position  1504  unless the button is compressed and to allow a turn of the selector from the third position  1504  through the second position  1502  to the first position  1500  without compression of the button. 
       FIG.  16    is a face on view of a selector  121  implemented with a risk button with the selector in an AUTO OB position. In this implementation, if selector  121  is moved to the SAFE position as shown in  FIG.  17   , the risk button plunger  908  must be depressed to move selector  121  into the SEMI CB position as block  1402  of plunger  908  is engaged with block  1302  of limited motion disk  906 . 
       FIG.  18    is a face on view of a selector  121  implemented with a risk button with the selector in a SEMI CB position. In this implementation, if selector  121  is moved to the SAFE position as shown in  FIG.  19    (from the SEMI CB position of  FIG.  18   ), risk button plunger  908  will start to ramp over the limited motion disk  906  as shown. If the selector  121  is moved further towards the AUTO OB position, it will pass the blocks  1302  on the limited motion disk  906  and risk button plunger  908  will have to be depressed to return to the SEMI CB position. As understood by one skilled in the art, the risk button assembly may be on the selector or located separately on the receiver of the firearm and interact with the selector. Furthermore, the risk button assembly and the selector may be rotated, slid, pushed, and/or switched, for example, as a rotary, slider, button, and/or switch/lever, respectively, to select a desired mode of operation. 
     It is common training for soldiers to ease the bolt forward after changing magazines and the bolt carrier is being held rearward by the last round stop, this practice prepares the weapon to be fired without making much noise. In order to ease the bolt forward the user must pull the charging handle back until it pulls the bolt carrier off the open-bolt sear and is held back by the charging handle. The trigger is then pulled to lower the open-bolt sear so that the carrier can proceed forward while the charging handle is slowly moved forward by the operator. 
     However, for a firearm that has been operated in full auto open bolt mode and thus has a heated barrel, if the user tries to ease the bolt forward to a closed bolt position the gun will fire unexpectedly when the round is chambered into the hot barrel. When the firearm has been operating in semi-auto mode, this easing procedure is still a safe and acceptable practice because in semi-auto mode, the round that is chambered won&#39;t accidentally fire when the bolt is eased forward. It&#39;s important however that the operator needn&#39;t remember this, particularly in combat situations. Firearm  100  may therefore be provided, in some embodiments, with a mechanism blocks pulling of the trigger following full auto operation and allows pulling of the trigger for easing the bolt forward in other scenarios. 
     A trigger in a selective fire firearm may have various positions based on the operational mode and operation of the firearm. For example, as shown in  FIG.  20   , a trigger  200  (e.g., a trigger suitable for use in firearm  100  as described herein, may have forward positions  2002  and  2006  respectively for semi-auto and full auto modes of operation. Because the forward position  2006  for the full auto mode is further back than the forward position  2002  for the semi-auto mode, the pulled position  2008  for the full auto mode (e.g., the position that releases the open bolt sear in an AUTO OB mode) is further rearward than the pulled position  2004  for the semi-auto mode (e.g., the position that releases the hammer from the trigger sear). 
       FIG.  21    shows a perspective cutaway view of a firearm  100  having a mechanism for both allowing pulling of the trigger to position  2004  and preventing pulling of the trigger to position  2008  (which would release the open bolt sear) when the charging handle of the firearm is pulled, thereby preventing the operator from easing the bolt forward past the open bolt sear in full auto mode and reducing or eliminating the risk of accidental firing of the weapon. As shown in  FIG.  21   , a lever arm such as a trigger block lever arm  2104  may be provided. 
     Lever arm  2104  may have a first end portion  2106  and an opposing second end portion  2108 . Second end portion  2108  may extend over rear portion  2102  of trigger  2000  when charging handle  2100  is pulled back as in the configuration of  FIG.  21    due to interaction between first end portion  2106  and charging handle  2100 . The interaction between first end portion  2106  and charging handle  2100  is illustrated in the top cutaway views of  FIGS.  22  and  23   . 
     As shown in  FIG.  22   , when charging handle  2100  is in a forward position (e.g., not pulled back), first end portion  2106  of lever arm  2104  may be separated from contact with charging handle  2100  and a spring  2202  may bias second end portion  2108  away from rear portion  2102  of trigger  2000  preventing second end portion  2108  from affecting the operation of trigger  2000 . When charging handle  2100  is pulled rearward (e.g., in direction  2204 ), a cam surface  2200  on charging handle  2100  may contact first end portion  2106  pushing first end portion  2106  outward (e.g., in direction  2206 ). 
     As shown in  FIG.  23   , when first end portion  2106  is pushed outward by cam surface  2200 , spring  2202  may be compressed and second end portion  2108  may move in an opposite direction  2300  from a first position away from trigger  2000  (see  FIG.  22   ) into a second position over rear portion  2102  (see  FIG.  23   ) so that, in full auto mode, second end portion  2108  can prevent trigger  2000  from being pulled to position  2008  of  FIG.  20    while allowing trigger  2000  to be pulled to position  2004 . 
     In this way, a trigger block lever arm may be provided that tracts on a cam surface on the charging handle, so that when the charging handle is pulled back, the lever arm is pivoted so it blocks the trigger from being pulled so that the operator can&#39;t ease the bolt past the open-bolt sear and, since the full-auto trigger pull is beyond the semi-auto trigger pull, the lever arm only blocks the trigger from being pulled while the firearm is in full auto mode and not while it is in semi-auto mode. 
     In one or more embodiments, the upper receiver of firearm  100  may have a forward assist button  2400  as shown in  FIG.  24    which may be used to force the bolt carrier assembly forward if the assembly gets stopped out of battery. The button  2400  can be depressed by the operator, which forces a plunger forward that engages notches cut on the bolt carrier and forces the bolt carrier forward. However, when the firearm is in a full auto open bolt mode of operation, if the trigger is pulled and the bolt carrier assembly gets stopped before firing a round and the operator uses the forward assist to force the bolt carrier forward, once the bolt carrier assembly reaches its locked position the round may fire whether the trigger is still pulled or not. Once the round fires the bolt carrier will recoil rearward with high energy and force the depressed forward assist plunger rearward into the hand of the operator causing possibly injury. 
     An optional-use cover such as cover  2500  of  FIG.  25    may be provided for the forward assist button  2400  that, when installed on the button  2400 , prevents an operator from pushing the forward assist button. Cover  2500  may be configured to be easily installed and removed from button  2400  without specialized tools and may have a surface that abuts an outer surface of a portion  2502  of the upper receiver to prevent button  2400  from being pushed. For example, the operator of firearm  100  may install the cover  2500  on button  2400  (e.g., by snapping the cover onto the button) when using the firearm in full auto open bolt mode.  FIG.  26    shows a side view of firearm  100  with cover  2500  installed on the forward assist button  2400 . 
       FIG.  27    shows a perspective view of cover  2500 . As shown in  FIG.  27   , cover  2500  may be formed from structure  2701  that defines a cavity  2700  configured to receive forward assist button  2400 . Structure  2701  may be formed from plastic, rubber, metal, ceramics, or other suitable materials. In one suitable example, structure  2701  may be a molded plastic structure. Structure  2701  may include an opening  2702  on one side and may be formed from a resilient material that allows button  2400  to be pushed into cover  2500  via opening  2702  and to snap onto button  2400  once button  2400  is within cavity  2700 . Structure  2701  may be provided with additional features such as slot  2704  and/or opening  2706 . Slot  2704  may facilitate stretching of cover  2500  onto button  2400  (e.g., by reducing the force needed to stretch opening  2702  during installation of cover  2500  and reducing the risk of breaking cover  2500  during installation and/or removal). 
     In accordance with one or more embodiments of the present disclosure, an open bolt sear assembly for a firearm, such as firearm  100 , may be provided that can be used to increase the reliability of the firearm by preventing possible misfires. For example,  FIGS.  28 A and  28 B  show exemplary configurations of an open bolt sear assembly engaged and disengaged, respectively, with a bolt carrier in accordance with an embodiment. 
     The open bolt sear of the open bolt sear assembly may permit the maintaining of desired performance aspects, while keeping a lower receiver compatible with a contemporary assault rifle (e.g., standard issue military AR). For example, a pivot location for the open bolt sear arm provides balance between adequate sear engagement and trigger pull travel, as discussed further herein. 
       FIG.  28 A  shows an open bolt sear assembly  2800  engaged with bolt carrier  224  according to an embodiment. Open bolt sear assembly  2800  (also referred to as a “dual pivot open bolt sear” or “dual sear pivot” herein) may be used when the selector (e.g., selector  121 ) is on AUTO OB mode. As shown in  FIG.  28 B , open bolt sear assembly  2800  includes an open bolt sear  2802 , a sear arm  2830  with a first end  2822  and a second end  2828 , and other individual components as discussed herein. Open bolt sear assembly  2800  illustrates a dual pivot open bolt sear, such that, for example, open bolt sear assembly has two pivot locations: sear arm pivot  2808 ; and open bolt sear pivot  2816 . For ease of fabrication, sear arm  2830  may be manufactured as two separate sear arms, such as sear arms  2804  and  2806 , that may be coupled together (e.g., at sear arm pivot  2808 ). In one or more embodiments, once coupled, sear arms  2804  and  2806  may operate as a single sear arm and are pivoted about sear arm pivot  2808  in unison. In another embodiment, sear arm  2830  may be manufactured as a single, monolithic sear arm. 
     In one or more embodiments of the present disclosure, the open bolt sear assembly  2800  may be used in a contemporary firearm, such as an M16 or M4 assault rifle, and/or other firearms such as, for example, firearm  100 . In an embodiment of the present disclosure, open bolt sear assembly  2800  allows open bolt sear  2802  to maintain adequate sear engagement with bolt carrier  224  under various circumstances. More specifically, a firearm using open bolt sear assembly  2800  may pass a drop test or a substantial impact to the firearm that may otherwise cause the firearm to discharge unexpectedly (e.g., slamfire). 
     The possibility of a misfire resulting from dropping or jarring a firearm may not be solved by a feature such as a stronger main spring without resulting in a substantial increase in trigger pull resistance. The dual sear pivot  2800 , however, may prevent misfires without substantially increasing the trigger pull resistance. 
     Open bolt sear assembly  2800  may hold the bolt carrier group (BCG) (e.g., bolt carrier  224  and bolt  225 ) in place when engaged with open bolt sear  2802  without resulting in a substantially high trigger pull resistance (i.e. trigger pull weight). The pivot location for the open bolt sear arm provides balance between substantial sear engagement and trigger pull travel. For example, a travel path  2812  of the open bolt sear  2802  from the open bolt sear arm pivot  2808  location illustrates how open bolt sear  2802  must force bolt carrier  224  rearward (e.g., toward the firearm stock) and compress main drive spring  402  before it releases bolt carrier  224 . The additional resistance necessary to force bolt carrier  224  rearward significantly raises the trigger pull weight. However, the open bolt sear assembly  2800  provides a second pivot point, open bolt sear pivot  2816 , relatively in-line with a contact point  2818  between bolt carrier  224  and open bolt sear  2802 . Pivot point  2816  thus creates a more vertical travel path (denoted by path  2810 ) of bolt sear  2802  and reduces the rearward travel of bolt carrier  224 , compression of main drive spring  402 , and thus the weight of the trigger pull. Furthermore, the relatively in-line pivot location at  2816  reduces the moment arm, which reduces the force and stress on open bolt sear  2802 , increasing reliability of the firearm. 
       FIG.  28 B  shows open bolt sear assembly  2800  disengaged from bolt carrier  224  of a firearm in accordance with one or more embodiments of the present disclosure. Open bolt sear assembly may be disengaged from bolt carrier  224  by, for example, a user pulling a trigger  2814  (e.g., rear portion  2820  of trigger  2814  may engage or lift first end  2822  of sear arm  2830 ). As understood by one skilled in the art, various triggers, such as triggers  202  and  702 , may also be used with open bolt sear assembly  2800 . In the disengaged configuration, open bolt sear  2802  is disengaged from bolt carrier  224  by sear arm  2830  pulling downward on open bolt sear  2802 , resulting in open bolt sear  2802  pivoting at open bolt sear pivot  2816 . Open bolt sear pivot  2816  is provided by engagement surfaces  2901  (see  FIGS.  29 A-D ) of legs  2900  contacting opposing inner surfaces of a lower receiver, such as lower receiver  102  of firearm  100 . Thus, engagement surfaces  2901  abut the interior surfaces of the lower receiver and open bolt sear  2802  is securely wedged between opposing inner surfaces of the firearm. Slot  2904  may receive a portion of sear arm  2830  to couple open bolt sear  2802  and sear arm  2830 . 
     For example, when trigger  2814  is pulled by a user, sear arm  2830  rotates at pivot  2808  (as indicated by directional arrows  3004  and  3006 ) and pulls body  2908  of open bolt sear  2802  in the direction indicated by arrow  3000  (e.g., relatively downward or away from the bolt carrier group), disengaging bolt carrier  224  and open bolt sear  2802 . As a result, bolt carrier  224  is forced in the direction indicated by arrow  3002  (e.g., forward or toward the barrel) by main spring  402 . The bolt carrier group cycles and the firearm operates as described previously herein during the duration trigger  2814  is held by the user in AUTO OB mode. Once trigger  2814  is released, sear arm  2830  is rotated (e.g., ends  2828  and  2822  move in directions relatively opposite of arrows  3004  and  3006 , respectively) by, for example, a spring. As a result of the rotation of sear arm  2830 , open bolt sear  2802  is pushed relatively upward in an opposite direction of arrow  3000  and at least partially into the travel path of bolt carrier  224 . Bolt carrier  224  is then stopped by open bolt sear assembly  2800  as mentioned herein and full automatic firing is ceased until trigger  2814  is pulled again. 
       FIGS.  29 A-D  show various views of open bolt sear  2802  in accordance with one or more embodiments of the disclosure. Open bolt sear  2802  has an elastic wishbone design (e.g., a substantially Y-shaped frame  2910  formed by body  2908  and legs  2900 ). More specifically, open bolt sear  2802  acts like a spring and dampens the impact force from bolt carrier  224  to prevent misfires. 
     A contemporary, fully automatic AR rate of fire may, for example, range 750 to 1200 rounds per minute cyclic; the bolt carrier travels back and forth with each cycle. In one or more embodiments of the present disclosure, when the trigger is released mid-fire, open bolt sear assembly  2800  is spring loaded to move into the bolt carrier&#39;s travel path and block the bolt carrier from cycling forward. The bolt carrier may impact open bolt sear  2802  with a substantial amount of momentum and force. The bolt carrier impacts open bolt sear  2802  on an impact surface (e.g., mating face  2902 ). The wishbone design allows legs  2900  to flex outwards during impact, as indicated by arrows  2906  of  FIG.  29 A , thus dispersing the impact force over more time which reduces the peak force and stress. The forces are thus directed into the receiver (e.g., lower receiver  102  of firearm  100 ), rather than passing the load through a retaining/pivot pin or other parts used in conventional open bolt sear assemblies. If the forces and stress exerted on the open bolt sear by the bolt carrier are not dampened, the contact of the open bolt sear and the bolt carrier may cause the open bolt sear to oscillate, which may result in the bolt carrier passing and riding over the open bolt sear and possibly firing one or more rounds. 
     The various embodiments discussed herein may provide various advantages for safe and efficient use of a firearm, particularly a selective fire firearm having a fully automatic open bolt mode of operation and a semi-automatic closed bolt mode of operation. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention may be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 
     Features may be removed, disabled, or not used in any desired type of firearm described herein. Thus, the features describe in conjunction with each type of firearm may be mixed and matched as desired and are by way of example only, and not by way of limitation. 
     Embodiments described above illustrate, but do not limit, the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.