the present invention is equally applicable to manual slide - operated firearms and gas - operated semiautomatic firearms of the type having a rotary bolt . since the fire control system and the reloading system of the firearm are not directly concerned with the firing pin block of this invention , and may be of various designs well known in the art , these systems have been omitted from the drawings . reference is made to the aforementioned u . s . pat . nos . 2 , 685 , 754 and 2 , 675 , 638 for typical examples of firearms to which the present invention might be applied . fig1 and 2 show a firearm having a barrel 12 formed with a cartridge - receiving chamber 18 . a barrel extension 14 is threaded on the barrel at 13 , and is formed with a series of conventional circumferentially - spaced locking lugs 16 . the barrel extension is attached by a bolt 19 to a forward portion 21 of a receiver 20 , which has a socket 23 at its rear end for mounting a shoulder stock ( not shown ). an action bar assembly 22 is reciprocably slidable in the receiver , and extends forwardly for attachment to conventional manual or gas operating means ( not shown ) located at the fore - end of the firearm . the action bars 22 are interconnected at their rear ends by an integral u - shaped yoke 24 , which fits into a recess 27 in a bolt carrier slide 28 to provide a fixed connection therewith . a resilient plastic ring 53 maintains correct lateral alignment between the yoke 24 and slide 28 . a breech bolt 30 is slidably and rotatably received in a bore 29 in the bolt carrier 28 . the bolt has an axial bore 32 , in which a firing pin 34 is reciprocably slidable between a retracted position shown in fig1 in which its tip 35 is withdrawn behind the bolt face 36 , and an extended position in which the tip protrudes from the bolt face , as shown at 35 &# 34 ; in fig6 to fire a cartridge ( not shown ) received in the chamber 18 . the firing pin is biased rearwardly by a compression spring 54 received in the bore 32 and bearing against shoulders 56 and 58 , respectively formed in the bolt and on the firing pin . the firing pin is secured by a retaining pin 60 , extending transversely through a cylindrical shank portion 42 at the rear of the bolt , and cooperating with a flat 62 milled on the firing pin to limit rearward movement of the firing pin relative to the bolt . forward movement of the firing pin relative to the bolt is limited by engagement of the shank 42 with an enlarged circular head or abutment 48 formed on the firing pin . ( see fig7 ). the distance that the firing pin tip 35 protrudes from the bolt face 36 is critical to proper functioning of the firearm . control of this dimension in the manufacture of the improved firearm is greatly facilitated by the fact that the engaging surfaces of the shank 42 and abutment 48 are flat , and parallel to the bolt face 36 . the bolt 30 is formed on its exterior cylindrical surface with a pair of similarly - shaped circumferentially - spaced cam slots 52 , of which only one appears in the drawings . these cam slots cooperate with a pair of cam pins 50 secured in the bolt carrier 28 to produce rotation of the bolt when it is moved axially relative to the bolt carrier . one of these cam pins may comprise the inner end of a manual bolt - operating handle ( not shown ), if desired . in the positions of the bolt carrier shown in fig1 and 3 , the bolt is rotated to an unlocked position , in which it protrudes forwardly from the bolt carrier , by movement of the cam pins 50 to the rear ends of the slots 52 . the bolt has a series of circumferentially - spaced locking lugs 40 around its outer surface ; as shown in fig2 these lugs are aligned to enter freely between the lugs 16 of the barrel extension 14 as the bolt is moved forwardly by the action bars 22 and bolt carrier 28 between the positions of fig1 and 3 . this angular alignment is maintained during this forward movement by a rectilinear channel 33 formed in the top of the receiver 20 , and extending parallel to the longitudinal axis of the bolt , for receiving and guiding one of the lugs 40 . the carrier 28 , which is guided for rectilinear reciprocation primarily by the action bars 22 riding on rails 25 formed in the side walls of the receiver 20 , also has a guide lug 41 that is slidably received in the channel 33 . the rearwardly - extending tubular shank portion 42 of the bolt 30 is slidably received in a bore 44 formed in a rearward extension 46 of the carrier 28 . the enlarged head or abutment 48 engages the carrier extension 46 to prevent forward movement of the firing pin 34 , in the relative positions of the parts shown in fig1 and 3 . the bolt 30 is shown in fig1 protruding forwardly from the carrier 28 to the maximum extent , as defined by engagement of the cam pins 50 with the rear ends of the cam tracks 52 . this protruding relationship is maintained , as the carrier moves the bolt forwardly , by the lug 40 sliding in the channel 33 . the maximum retraction of the bolt into the carrier , as shown in fig7 is defined by the cam pins 50 engaging the forward ends of the cam tracks 52 , after these cam means have rotated the bolt to its locked position . a hammer 66 is pivotally mounted on a pin 68 secured in the receiver 20 , and is rotatable clockwise , when released by a suitable trigger and fire control mechanism ( not shown ) from a cocked position shown in fig1 to a firing position shown in fig7 in which it impacts the rear end of the firing pin to discharge the firearm . a box magazine 70 is mounted in the receiver in a conventional fashion to supply a series of cartridges , each of which is fed successively into the chamber 18 by a forward movement of the bolt 30 . in the illustrated embodiment , the clockwise rotation of the hammer 66 is limited by its engagement with the magazine 70 , as shown in fig7 and the firing pin is driven forwardly by the momentum of hammer impact imparted to it prior to this engagement . referring now to fig3 the bolt carrier 28 and action bars 22 are shown moving forward , in the direction shown by the arrow . the bolt has passed over the magazine 70 , where it could have picked up a cartridge ( not shown ), and its forward stroke has been terminated by abutment against the barrel 12 , in which position it would have seated the cartridge in the chamber 18 . the bolt lugs 40 have passed freely between the barrel lugs 16 , have left the receiver channel 33 , and have entered a cylindrical recess 17 in the barrel extension 14 , so that the bolt is now free to rotate . however , no bolt rotation has yet occurred , since the cam pins 50 have not yet been moved by the forward travel of the bolt carrier 28 from the rear ends of the cam tracks 52 . it will be observed in fig3 that a premature release of the hammer 66 , which is possible at this point , will merely impact the head 48 of the firing pin against the rearward extension 46 of the bolt carrier , since these parts still remain in contact . thus the firing pin is blocked and cannot be moved forwardly in the bolt bore 32 at this time , either by impact of the hammer 66 , or by its own inertia if the firearm is accidentally dropped , even if the spring 54 should be distorted , broken , or missing . as shown in fig4 the action bars 22 have carried the bolt carrier 28 far enough forward relative to the bolt 30 , which can travel no farther , to cause the pins 50 to traverse the curved portions and enter into the longitudinal portions of the cam slots 52 , and at the same time to move forward a distance l relative to the bolt . this distance is slightly more than sufficient to rotate the bolt and engage the bolt lugs 40 behind the barrel lugs 16 , as shown in fig5 to lock the bolt to the barrel extension 14 . the firing pin 34 , being pressed rearwardly by its spring 54 against the retaining pin 60 in the bolt , begins to protrude rearwardly from the carrier extension 46 at the same time as the carrier 28 commences to move forward from the fig3 position toward that of fig4 . the carrier 28 consequently reaches a position in fig4 such that the gap between the head 48 of the firing pin and the carrier extension 46 is equal to l , the same distance that the cam pins 50 have moved forwardly in the bolt 30 . the parts are so dimensioned that the distance between the tip 35 of the firing pin and the bolt face 36 is also equal to l . if the trigger is pulled at the stage shown in fig4 releasing the hammer 66 to strike the firing pin at a position 66 &# 39 ;, it will be seen that the head of the firing pin can travel forwardly only the distance l before it is stopped at a position 48 &# 39 ;, abutting against the carrier extension 46 . therefore the firing pin tip will be halted at a position 35 &# 39 ; flush with the bolt face 36 , so that it cannot discharge a cartridge in the chamber . it would be necessary for the firing pin to travel an additional distance forward before the tip 35 could protrude far enough from the bolt face to indent and ignite the primer of a cartridge . thus , even though the bolt has reached a fully - locked condition at the stage shown in fig4 still further forward travel of the bolt carrier is required before the firearm can be discharged ; this provides an additional margin of safety to cover the possibility that the parts might become distorted or badly worn . in the position of fig4 the carrier extension lies a distance i behind the bolt shank 42 . additional forward movement of the carrier 28 through the distance i is illustrated in fig6 . if the hammer is released at this stage , it will drive the firing pin forward the distance l + i , causing the head to reach a position 48 &# 34 ; abutting against the bolt shank 42 , and the tip to reach a position 35 &# 34 ; protruding the distance i beyond the bolt face 36 . this is the maximum depth to which a cartridge primer can be indented by the forward momentum of the firing pin . the cam tracks 52 have straight lost - motion sections extending forwardly from their curved sections a dwell distance d , as appears in fig4 and 6 , to allow not only the incremental forward movement i of the carrier 28 and action bars 22 , but also some additional movement into their extreme forward positions shown in fig7 . at any point in this additional movement , the weapon may be fired by releasing the hammer , which strikes the head of the firing pin as it reaches a position 66 &# 34 ;, in which it is halted by contact with the magazine 70 . the momentum imparted to the firing pin 34 compresses the spring 54 , and drives the firing pin forwardly until its head strikes the bolt shank 42 at the position 48 &# 34 ;, and its tip projects from the bolt face at the position 35 &# 34 ; to discharge the firearm . fig8 illustrates the initiation of subsequent movements of the parts to reopen the breech and recock the hammer . the action bars 22 are driven to the rear , as shown by the arrow , by either manual or gas operation as the case may be . this initially retracts only the bolt carrier 28 , as its attached cam pins 50 pass through the straight lost - motion portions of the cam slots 52 . then the cam pins pass into the curved portions of the cam slots , rotating the bolt 30 to the unlocked position of fig2 . at the illustrated stage , with the cams pins 50 at the rear ends of the slots 52 , the rearward movement of the action bars and bolt carrier is imparted to the bolt 30 as well . during the rearward travel of the bolt carrier 28 , its extension 46 pulls the firing pin head 48 along with it , away from contact with the bolt shank 42 . the head 48 soon engages the hammer 66 , and pushes it counterclockwise out of contact with the magazine 70 . the completion of the counterclockwise hammer recocking motion is effected by a subsequent engagement between the bottom of the bolt carrier and the hammer , until the parts reach the position of fig1 when the hammer is held cocked by the aforementioned fire control . in the preferred embodiment which has been illustrated and described , the rearward extension 46 of the bolt carrier 28 is made even longer than is necessary to ensure that the bolt lugs 40 must be fully locked to the barrel lugs 16 before the firing pin tip 35 can be made to protrude from the bolt face 36 to discharge the weapon . this provides a margin of extra safety in the event that the parts , particularly the enlarged firing pin head or abutment 48 and the extension 46 , might become worn or distorted . this aspect of the invention can be more clearly understood by analyzing the dimensional relationships of the parts in somewhat greater detail . as appears in fig6 and 7 , the length of the firing pin 34 , between the forward surface of the abutment 48 and the tip 35 , is equal to the length of the bolt 30 plus the maximum cartridge indent distance i , which is reached by the tip upon contact of the abutment with the bolt shank 42 . the extension 46 is of such a length that after the bolt carrier has moved forwardly from the position of fig1 to that of fig3 to close the bolt against the breech of the barrel 12 , it holds the abutment 48 at a distance l + i to the rear of the bolt shank 42 , and consequently holds the firing pin tip at a distance l to the rear of the bolt face 36 . movement of the bolt carrier farther forward a distance l , to the position of fig4 causes the cam pins 50 to pass through the curved portions of the cam tracks 52 , whose longitudinal length defines the distance l , and rotates the bolt to lock it to the barrel . at this stage , the carrier extension 46 still lies to the rear of the shank 42 a distance i , so that the abutment 48 can travel forwardly no more than the distance l , only enough to bring the tip 35 flush with the bolt face . still further forward travel of the bolt carrier through the distance i , to the position of fig6 is necessary to remove the block against contact between the abutment 48 and the shank 42 . it will be seen from this analysis that if the extension 46 were to be shortened by the length i , the full bolt lock - up condition shown in fig4 would still have to be achieved before the abutment 48 could contact the shank 42 and cause the tip 35 to protrude the distance i . if the extension 46 were to be shortened still more , this protrusion of the firing pin could be permitted at some point during the locking rotation of the bolt 30 , when the lugs 40 and 16 were sufficiently engaged to insure safety , but not fully interlocked as shown in fig4 and 5 . such variations are considered to fall within the scope of the invention in its broader aspects , although they are not preferred because of the lesser degree of safety they would provide in a firearm that is intended to serve a useful life of indefinite length , and whose parts might become worn or distorted after long use .