LIGHT MACHINE GUNS A light machine gun is a truly automatic (self-firing) fire-arm which is portable but which is too heavy to be adapted for use as a close-combat weapon. The increased weight (17-28 lb.) makes the recoil energy less than that of the rifle, and as a rest is provided at the front end the continuous fire delivered remains reasonably accurate under ordinary conditions. To make the best use of this weapon much ammunition must be carried and it is accordingly manned by a team, one man carrying the gun and the others ammunition, spares and accessories.
All these weapons are operated either by the gas or by the recoil generated by the charge, and all are now air cooled. All gas-operated weapons are practically identical in principle and where differences occur they are usually only in matters of detail. Usually, the gas is diverted through a hole in the barrel into a cylinder, where it strikes the bead of a piston and drives it to the rear. Towards the rear of the piston are embodied devices which connect it to the "feeding" arrangements and also to those for sealing the breech and firing the round. The piston, when driven to the rear, compresses a spring which, baying absorbed the thrust, returns it to its forward position. As the piston travels to the rear the following operations are performed— extraction and ejection of the fired round, and the presentation of the new round ready for insertion into the chamber. When the spring forces the piston forward, the bolt (mounted on the piston) drives the round into the chamber. During this move ment, locking devices come into play and lock the bolt to the breech,, thus sealing it and supporting the bolt against the ex plosion. This occurs before the finish of the piston's travel, so that the striker, carried on the piston, only strikes the cartridge cap after locking has taken place, and the weapon is accordingly "mechanically safe." Unlocking takes places after the piston
has been driven a short distance to the rear, by which time the bullet is out of the barrel. In this respect also the weapon is "mechanically safe." A typical gas-operated light machine gun is the Lewis (see fig. 4) or Browning.
In recoil operated light machine guns, the mechanical systems , employed are more varied than ,?ro those used in gas-operated weapons, and there is no general principle to which they closely adhere. A feature common to all, however, is the recoiling barrel which overcomes the resistance of antagonistic springs. These, at the finish of its backward travel, return the barrel to its forward position. The length of recoil varies in different guns from about i in. to 5 in. ; consequently they are classified as "long" or "short" recoil. (The length of the recoil is shown in the table below.) Another common feature is that the barrel and bolt (or lock) are mechanically locked together for a certain length of the recoil, thereby sealing the breech until the bullet is clear of the barrel. In "long recoil" actions, the bolt, having come back with the barrel, is held in its rearmost position, with the extractor engaged behind the rim of the cartridge which is still in the chamber. Accordingly, as the barrel moves forward under the influence of the springs which have been overcome in the backward travel, the round is withdrawn from the breech and ejected. The Chauchat, now obsolescent in the French army but adopted in that of Belgium, is a type of the "long recoil" gun.
In "short recoil" actions, such as that of the Bergmann gun of approximately one inch barrel movement, a form of mech anism is provided whereby the bolt, locked to the barrel for a certain length of recoil (created by firing) is unlocked and with drawn about 3 in. from the barrel after the latter has completed its inch of recoil. In this separation of bolt and barrel, extraction and ejection of the empty case is effected, and a new round is inserted into the chamber. The Madsen action does not conform to the above description, as in this case the separation of breech from bolt is effected on recoil by guiding the bolt away from the barrel in a vertical plane, the bolt being pivoted in rear to permit this movement.