Plate 23.-Bolt action, Browning automatic rifle, model of 1918.

nism when a shot is fired. These two phases are the backward (first phase) and the forward action (second phase). In making the division the igniting of the cartridge in the chamber is the dividing point.

Plate 24.-Trigger mechanism, Browning automatic rifle, model of 1918. 15. First phase.-a. Action of gas.-A cartridge having been ignited, the bullet under the pressure of the expanding powder gases travels through the barrel, and when it reaches a point 6 inches from the muzzle, it passes a port in the bottom of the barrel. The barrel pressure, which at this instant is still very high, seeks this first natural vent. Registered with the barrel port are other similar ports in the gas cylinder tube bracket, gas cylinder tube, and gas cylinder. The port in the gas cylinder is the smallest and serves to throttle the barrel pressure. The ports in the gas cylinder lead radially into a well about % inch in diameter in the head of the gas cylinder. The throttled barrel pressure is conducted through this well to the gas piston plug. This pressure acts on the piston a very short time, namely, the time it takes the bullet to travel the 6 inches distance from the barrel port to the muzzle. Its effect is that of a sudden severe blow on the piston plug. Under the influence of this blow the gas piston is driven to the rear and carries with it the slide to which it is assembled. When the piston has traveled about 9/16 inch backward, the bearing rings on its head, also the

gas piston plug, pass out of the cylinder. The gas expands around the piston head and into the gas cylinder tube, and is exhausted through 6 portholes in the tube, just in rear of the gas cylinder tube bracket. The gas is prevented, in a large measure, from traveling back through the gas cylinder tube by two rings on the piston, about % inch apart and 11⁄4 inches from the piston head. These rings also serve as bearings to hold the front end of the piston in the center of the gas cylinder tube after the piston head has passed out of the gas cylinder.

b. The slide. As the piston is forced back as described above it carries with it the slide. The first and immediate result of the backward movement of the slide is the beginning of the compression of the recoil spring, thereby

SHOWING EXPANDING GASES PASSING THROUGH GAS PORT INTO GAS CYLINDER. GAS
STRIKES PISTON PLUG WITH HAMMER LIKE BLOW DRIVING PISTON WITH SLIDE TO THE REAR

SHOWING ACTION OF GAS, MOVEMENT OF PISTON TO REAR AND

GAS ESCAPING THROUGH SIX HORT HOLES IN THE TUBE

SHOWING PISTON IN ITS REAR MOST POSITION

Plate 25.

storing energy for the forward action.

c. Unlocking.-The hammer pin is slightly in advance of the bolt link pin, about 0.19 of an inch. The center rib of the hammer is very slightly in rear of the head of the firing pin. When the slide begins its motion to the rear it imparts no motion whatever to the bolt and bolt lock. The slide moves back 0.19 of an inch and its only effect during this travel is to carry the hammer from the firing pin and the hammer pin directly under the bolt link pin. At this point the unlocking begins, the bolt link revolves forward about the hammer pin, drawing the bolt lock down and to the rear. The motion of the lock and bolt, which is zero at the instant the hammer pin passes under the bolt link pin, accelerates from this point until the slide has traveled about 1.2 inches, at which point the bolt lock is drawn completely down out of the locking recess and away from the locking shoulder of the reeciver. It is now supported in front of the bolt supports, and the front upper shoulder of the bolt link has revolved forward and bears upon

the locking shoulder of the bolt lock. These two influences prevent the bolt lock from revolving down below the line of backward travel of the bolt.

d. Withdrawal of firing pin.-As the bolt lock revolves down from its locked position, a cam surface in a slot in the rear bottom side of the bolt lock comes in contact with a similar cam surface on the firing pin lug, and cams the firing pin from the face of the bolt.

e. Extraction.-The backward motion of the bolt begins when the bolt lock has been drawn down so that the circular cam surface on its underside is operating on the rear shoulders of the bolt supporters. This produces a strong lever action which slowly loosens the cartridge case. The backward travel of the bolt has been slight, only about 5/32 inch when the firing pin is withdrawn; its travel is about 11/32 inch when the bolt lock is completely drawn down. From this point the bolt moves to the rear, drawn by the bolt lock and bolt link, with the same speed as the slide and carries with it the empty cartridge case, which is held firmly in its seat on the face of the bolt by the extractor. The extractor is on the upper right-hand side of the bolt next to the ejection opening in the receiver. A slot cut in the left side of the bolt lock near the back end passes over the bolt guide, which supports the bolt lock and bolt when they are in the rear position.

f. Ejection.-When the slide reaches a point about 4 inch from the end of its travel, the base of the cartridge case strikes the ejector, which is on the left side of the feed rib of the bolt, and opposite the extractor. This action causes the cartridge case to be pivoted with considerable force about the extractor, and through the ejection opening in the receiver. The front end of the cartridge case passes first out of the receiver, and is pivoted so that it strikes the outside of the receiver at a point about 1 inch in rear of the ejection opening. It rebounds from the receiver toward the right front.

g. Termination of first phase.--The backward motion is terminated when the rear end of the slide strikes the buffer. The slide moves forward 1/10 inch, after striking the buffer, under the action of the recoil spring, but if the sear nose is not depressed, it engages the sear notch on the slide, and the piece is cocked for the next shot.

NOTE. It is to be noted that the motion of the bolt, bolt lock, and bolt link mechanism began slowly at first and did not attain the speed of the slide until the latter had traveled about 1/4 inches backward. This is a very important characteristic of the rifle, since on this account the mechanism is not subjected to an excess strain due to a sudden start at the instant the gas impinges upon the piston. This slow start delays the opening of the chamber sufficiently to allow the high barrel pressure to decrease.

16. Second phase.-a. Action of recoil spring. The sear nose is depressed disengaging the sear, and the slide moves forward under the action of the recoil spring. The position of the bolt link pin is slightly below a line joining the bolt lock pin and the hammer pin; therefore as the slide starts forward the joint at the bolt fink pin has a tendency to buckle downward. It is prevented from doing this by the tail of the feed rib on the bolt which extends backward under the bolt lock, also by the upper front shoulder of the bolt link being in contact with the locking surface of the bolt lock. Since the joint can not buckle, the entire mechanism moves forward with the slide. When it has traveled about 4 inch, the front end of the feed rib impinges on the base of the top cartridge, which the magazine spring and lips are holding up in its path.

b. Feeding. The cartridge is carried forward about 4 inch, when the nose of the bullet strikes the bullet ramp or guide on the breech of barrel, and is deflected upward toward the chamber. This action also guides the front end of the cartridge from under the magazine lips. The base of the cartridge approaches the center of the magazine, where the lips are cut away and the opening enlarged, and at this point is forced out of the magazine by the magazine spring. The base of the cartridge slides across the face of the bolt and under the extractor. Should the cartridge fail to slide under the extractor, the extractor will snap over its head as the bolt reaches the forward position. When the cartridge is released by the magazine, the nose of the bullet is so far in the chamber that it is guided by the chamber from this point on.

c. Locking. When the slide is about 2 inches from its forward position, the circular cam surface on the under side of the bolt lock begins to ride over the rear shoulders of the bolt supports, and the rear end of the bolt lock is cammed upward. The bolt link pin passes up above the line joining the bolt lock pin and hammer pin. The joint at the bolt link pin now has a tendency to buckle upward, and the bolt lock being opposite the locking recess in the receiver is free to, and does, pivot upward about the bolt lock pin. The bolt link revolves upward about the hammer pin, forcing the bolt lock up, and a rounded surface on the bolt lock just above the locking face slips over the locking shoulder in the receiver, giving the lock a lever action which forces the bolt home to its final position. The two locking surfaces on the bolt lock and the receiver register as the hammer pin passes under the bolt link pin.

d. Igniting the cartridge.-The lug on the firing pin is buried in the slot on the underside of the bolt lock at all times except when the bolt is locked in the forward position. Therefore, the firing pin is locked away from the face of the bolt during all the backward and forward motion of the bolt. When the hammer pin passes under the bolt link pin, the firing pin has been released by the bolt lock. The slide and hammer move forward about 1/10 inch farther, and the center rib of the hammer strikes the head of the firing pin, driving it forward, and igniting the cartridge.

e. Termination of second phase.-The forward end of the slide strikes a shoulder at the rear end of the gas cylinder tube which terminates the forward motion. The forward motion is not terminated by the hammer on the firing pin. This can be seen by examining the head of the firing pin when the gas cylinder tube is assembled to the receiver, and the bolt mechanism is in the forward position. The firing pin has still about 1/16-inch clearance from its extreme forward position.

NOTE. The locking shoulder of the receiver is inclined forward. Its surface is perpendicular to the line through the bolt lock which the shock of the explosion follows; therefore, the force of this shock is exerted squarely against this normal surface. It should be noted that the speed of the bolt mechanism is slowed down gradually from the instant that the bolt lock starts to rise until the hammer pin passes under the bolt link pin, when its speed is zero.

17. Functioning of buffer.-a. The buffer system consists of a tube, in which are placed, successively from front to rear, the buffer head; a brass friction cup with concave interior which is split to allow for expansion; a

steel cone to fit into the cup. Four of these cups and cones are placed one after the other in series. In rear of these is the buffer spring, and finally the buffer nut, which is screwed into the end of the tube and forms a seat for the spring.

b. The action. The buffer head, struck by the rear end of the slide, moves to the rear, forcing the cups over the cones and causing them to expand tightly against the tube, consequently producing considerable friction as the cups move back and compress the buffer spring. Thus the rearward motion of slide is checked gradually and there is practically no rebound. The spring returns the buffer head and friction cups and cones to their original positions. 18. Functioning of trigger mechanism.-a. The trigger mechanism has three settings:

Showing trigger mechanism single shot firing. The connector cammed forward from under the forward end of the sear by the under cam surface of the sear carrier, permits the forward end of the sear to return to position under tension of sear spring, causing the rear end of the sear to engage in the sear notch of the slide.

Showing trigger mechanism set on safe. Cylindrical portion of the change lever resting over the heel of the trigger prevents the upward movement of the trigger and the releasing of the sear.

Plate 26.