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Part Of  Bert '53  History

Section II


a. The Browning machine gun cal. .30 (all types), and the cat 32 trainers are fundamentally automatic weapons which once in motion will function by their own inertia as long as ammunition remains in the belt. However, though automatic, it is necessary to manually load and lock the mechanism for the initial round of am- munition only, and to start its operating sequence by pressure upon the trigger. The functional description which follows is taken from the standpoint that these operations have been performed. The function of all guns is the same except where noted.

b. The automatic firrng of the gun begins with the action of the trigger, releasing the firing mechanism, which in turn strikes and fires the cartridge. Upon this action the burning powder within the car- tridge case violently generates gas As the gas is restricted to a rela- tively minute space by the cartridge case and bullet, a tremendous pressure is exerted. This pressure exerts itself against the rear face of the bullet, which at this time is still retained in the case. This driving force expels the bullet from its case and drives it through the barrel This same force attempts to drive the empty case out of the chamber in the rear end of the barrel This action is arrested by the holt which at the mstant of firing is locked against the rear of the cartridge.

c. The recoil drives the barrel barrel extension, and bolt rearward This movement unlocks the holt assembly from the barrel extension (allowing it to travel rearward) compressing the driving spring until the bolt is stopped by the buffer plate in the back plate. During this recoil motion, the bolt withdraws the tmpty cartridge case from the chamber and picks up another live cartridge from the loaded feed belt. The empty cartridge is expelled from the bottom of the gun by ejection, while the extractor positions the succeeding cartridge in the T-slot of the bolt. Meanwhile the barrel and barrel extension have been stopped and held by the accelerator of the lock frame group assembly acting upon the barrel extension shant Thus the mechanism is ready for reloading.

Figure 12 - M1919A4 Machine Gun -- Right Side Sectional View

d. The compressed driving spring at this moment expands and forces the bolt assembly forward. As the bolt assembly now travels forward, the barrel extension is released from the lock frame, and the barrel plunger spring, which was compreaed during the recoil motion, returns the barrel extension and barrel into firing position. Meanwhile, the bolt has been locked to the barrel extension and the barrel The firing pin is then released and strikes the contact point of the cartridge case. As the firing pin strikes the cartridge, another cycle is started, and succeeding cycles will continue as long as ammunition is supplied and trigger action is maintained.


a. The following description exempUfies the cyclic operations of a manually-exerted trigger-activated cal. 30 Browning machine gun (fig. 12). This functional description is identical with that of trigger bar, solenoid, or trigger motor except for the actual manner in which the firing pin is released. NOTE: The ground guns are fired by direct action of the trigger camming the sear down to disengaPe it from the firing pin. The trigger of the MZ Aircraft Gun (Aexible) acts upon a triBBer bar which presses up on the seer holder, which in turn cama the seer laterally to release the firing pin. The solenoids or tri8Ber motor act upon the seer directly to move it laterally.

b. Every cycle of the gun entails certain simultaneous movements or parts functions which must be performed in perfect relationship. In order that these functions be more fully explained, it ie deemed advisable that they receive individual treatment. These phases will be discussed in the following order: firing, recoiling, counterrecoiling, cocking, automatic firing, feeding, extraction and ejection.


(a) With the gun loaded and the firing pin spring cocked, the gun is ready for firing (fig. 13).

Figure 13 - Ready To Fire Position

(b) Upon the action of the trigger, the trigger cams on the front end engage with the came on the seer and force the tear down against the action of the sear spring until the shoulder of the firing pin is released from the sear notch (fig. 14). The Firing pin is then driven forward through the bolt by the firing pin spring, and the firing pin contact point strikes and fires the cartridge

Figure 14 - Firing Position


(a) The cycle of recoiling takes place as the cartridge is fired and the bullet is propelled from the end of the barrel (fig. 15). This causes a recoil stroke of the barrel, barrel extension, and the bolt. These groups of parts are commonly called the recoiling parts.

Figure 15 - Recoiling Parts in Forward Motion

(b) At the time of firing, the breech lock is holding the bolt securely against the cartridge and barrel (fig. 16), locking the barrel extension to the bolt. The breech lock is held in the locked position by the breech lock cam.

Figure 16 - Breach Lock in Locked Position

(c) When the cartridge explodes and the bullet is propelled from the harrel, the force of the recoil drives the recoiling parts rearward. Within the first part of the stroke the breech lock drops off the breech lock cam and releases the bolt from the barrel extension. The breech lock itself is forced down by the front projections of the lock frame acting on the breech lock pin. This takes place when the recoiling parts have traveled approximately three-eighths inch rearward (fig. 17). Thus the holt is then free to continue its travel to the rear.

Figure 17 - Breach Lock in Unlocked Position

Figure 18 - Recoiling of Barrel and Barrel Extension

(d) As the barrel extension moves rearward (fig. 18), the barrel plunger spring is compressed and the rear of the barrel extension strikes the accelerator and turns it backward. As the accelerator turns backward, its tips strike the bottom projections on the bolt and accelerate it to the rear At the same time the claws of the accelerator engage the shoulder of the T-lug on the barrel extension, locking it to the lock frame The accelerator stop limits the backward motion of the accelerator. The barrel plunger spring is thus held compressed (fig. 19).

Figure 19 - Barrel Plunger and Spring Compressed

(e) As the bolt is now paring freely in a rearward direction, the driving spring is being compressed. Meanwhile the boh has, by means of the extractor secured a fresh cartridge from the feed belt. The empty cartridge case is still held in the bolt T-slot.

(f) As the holt moves rearward, the upper portion of the cocking lever is forced forward in the cocking lever recess. The lower end, moving rearward, retracts the firing pin, thus compressing the firing pin spring against the sear spring pin. The shoulder of the firing pin engages the notch in the sear which is pulled upward by the sear spring The trigger cams are now disengaged from the sear.

Figure 20 - Recoil Completed

(1) The driving spring has been compressed and recoil completed (fig. 20) when the rear of the bolt strikes the buffer plate. The remaining rearward force is absorbed in the buffer plate and the buffer disks. A part of the recoil energy of the bolt is now stored in the driving spring assembly.

Figure 21 - Barrel Plunger and Spring Released


(a) After completion of the recoil stroke, the bolt is propelled forward by means of the energy which has been stored in the driving spring and the compressed buffer disks. When the bolt moves forward, the accelerator is struck by the projection on the bottom of the bolt. This rolls the accelerator forward and releases the barrel plunger spring (fig. 21).

(b) As the accelerator rolls forward, the accelerator claws are moved away from the shoulders of the barrel extension shank, Thus the barrel extension is free to move forward. This forward movement is accomplished by means of the expansion of the barrel plunger spring, assisted by the forward motion of the bolt acting upon the accelerator.

Figure 22 - Breach Lock Engaged

(c) As the barrel extension moves forward, the breech lock engages the breech lock cam and is forced upward. The bolt, which has been continuing its forward motion since striking the accelerator has at this instant arrived at a position where the notch on the under side is directly above the breech lock, thus the breech lock engages the bolt (fig. 22), and the bolt is thereby locked to the barrel extension, and hence the barrel, just before the recoding parts reach the firing position.

Figure 23 - Cocking Action During Recoil


(a) The act of cocking the gun is begun at the bolt moves in recoil. The upper end of the cocking lever is forced forward by en- gaging the top plate (fig. 23) or the top plate bracket which is attached to the top plate of the M2 Aircraft Guns This brings the lower end of the cocking lever to the rear.

(b) As the lower end of the cocking lever moves to the rear, it brings with it the firing pin. Thus the firing pin is withdrawn from the face of the bolt and the firing pin spring is compressed against the sear spring pin.

(c) As the bob moves in counterrecoil, the upper end of the cock- ing lever is again engaged by the top plate, This remover the lower end from the path of the firing pin The pin is ready to strike when it is released from the sear.

Figure 24 - Automatic Firing


(a) Automatic firing is accomplished by pulling the trigger which depresses the front end, and then holding it thus in a firing position (fig. 24). The sear is depressed when its camming surface engages cam surfaces of the trigger during the forward movement of the bolt near the end of the counterrecoil stroke Depression of the sear releases the firing pin from the notch at the bottom of the sear. The firing pin is then driven forward by the expanding firing pin spring and automatically fires the next cartridge at the end of the forward stroke.

(b) The weapon will continue to fire automatically, at long as the trigger is held in a firing position, until the supply of ammunition fed to the gun is exhausted.

Figure 25 - Action of Belt Feed Mechanism
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(a) The belt feed mechanism is actuated by the bolt (fig. 25). When the bolt is in its forward position, the belt feed slide is within the confines of the gun. The lug on the end of the belt feed lever is engaged in the diagonal cam groove in the top of the bolt.

Figure 26 - Action of Belt Feed Pawl
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(b) As the bolt moves rearward during recoil, the belt feed lever is actuated so that the forward end of the belt feed lever moves the belt feed slide out of the side of the casing and over the ammunition belt where the belt feed pawl then engages the next cartridge for feeding into the feedway (fig. 26). The ground guns are fed from the left side. NOTE: The M2 Aircraft Gun is adaptable for feeding from either right or left side.

(c) As the bolt moves forward in counterrecoil, the belt feed lever is further actuated by the cam groove in the top of the bolt, so that the belt feed slide and the belt feed pawl are moved laterally.

(d) The belt feed pawl then carries the first cartridge against the cartridge stops. This new round is thus ready to be gripped by the extractor.

(e) The next cartridge is carried over the belt holding pawl which rises behind it and holds it in position to be engaged by the belt fed pawl in the next cycle.

Figure 27 - Extracting Cartridge at Beginning of Recoil
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Figure 28 - Beginning of Breach Lock Unlocking
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Figure 29 - Cartridge Entering T-slot
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Figure 30 - Ejecting Fired Cartridge Case
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Figure 31 - Chambering Cartridge
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