Light Machine Guns

In "short recoil" guns, the recoil and the return of the barrel is the source of power for operating the mechanism which brings about the feeding of the cartridges into the gun, in the same way as does the piston in gas-operated weapons. Normally, with such guns the rounds are contained in a belt which is mechanically led through the gun, the rounds being withdrawn from it during its passage. In "long recoil" weapons, the system of feed is usually by magazine, the rounds being pushed by a spring inside the magazine into the space which is created when the bolt re mains back as the barrel goes forward.

The Sub-machine Gun.

The sub-machine gun is a kind of automatic pistol, but heavier, more powerful, and with a longer range than the usual pattern of pistol. It is capable of a high rate of fire and is lighter than other automatics. The ammunition is similar to pistol ammunition. In the Thompson sub-machine gun the breech is sealed by the agency of inertia together with the adherence of inclined surfaces to those portions of the mechan ism which support the bolt against the gas pressures generated within the barrel.

Some of the principal light machine guns employed during the World War and some of the newer types are tabulated below with a few details.

Locking Actions.

As the locking action is perhaps the most important part of the mechanism of any automatic weapon, a short description is given of the well-known method employed in the Vickers gun and also the unusual methods employed in the Schwarzlose (see fig. 6) and Revelli guns.

The Vickers.—The lock is connected to a crank by (I) a side lever head, whose axis we shall call a, and (2) a connecting rod, whose axis we shall call b. The axis of the crank we shall call c. In the locked position, axis b is slightly below axis a and c, and the crank, under b, bears on the crank stops. The rearward thrust of the cartridge on explosion thus passes through a, b and c. As b is lower than a and c, it cannot under pressure rise above the line ac and so unlock the action. It must tend downwards. However, the crank is already bearing on the crank stops under b, hence no movement here is possible and so the locking action is effected.

Except that the crank revolves downwards instead of upwards, the system of locking in the Maxim gun is practically identical with that employed in the Vickers.

Inertia Sealing of the Breech in the Schwarzlose.—There is no positive locking in the gun, reliance being placed in heavy moving parts and a strong spring, to induce sufficient inertia to seal the breech. With a view to ensuring the exit of the bullet from the barrel before the inertia is overcome, the barrel is abnormally short.

The mechanism includes a massive, heavy breech block a, connected by a crank link to the upper axis of a crank. The lower axis of the crank works in bearings in the body-casing of the gun and has no backward or forward movement. In rear is a strong spring pushing the bolt forward. The thrust on the face of the breech block pulls the crank link to the rear. It, in turn, pulls the crank to the rear.

However, owing to the set of the crank in relation to the lower non-recoiling axis, much resistance to the rearward motion of the breech block is set up. Thus it is that, coupled with the heavy resistance offered by the strong antagonistic spring and the weight of the moving parts, sufficient inertia is created to prevent the breech from opening prematurely.

The Revelli.—On explosion, a barrel with a sleeve a recoils, and the gases, acting through the cartridge head, impart a rearward movement to a breech block b. At this stage, the breech block and sleeve are held together by a claw, on a rotating wedge. The rearward movement of the Revelli breech block causes the wedge to rotate to the rear. In so doing, the claw, which passes through a slot in the sleeve, bears against the latter at the shoulder, and helps to force the sleeve and barrel rearward. After a and b have moved about 4 mm., the claw is entirely disengaged from the breech block, which continues to the rear under its own momentum. The wedge in rotating compresses a strong spring, so the tip of the claw bears against the under side of the breech block, and is prevented from regaining its forward position. Hence the sleeve cannot do so until the recess in the breech block returns to within a mm. of its forward position.