MACHINE GUN, a firearm of small cali bre fitted with mechanism by which it may be fired successively with great rapidity. Two dis tinct classes have been developed: (1) That in which the firing is done by turning a crank by hand; and• (2) an automatic form which util izes the recoil of the gun, or a part of the force of the exploding charge to operate the firing and reloading mechanism. The first class has been practically abandoned. In the automatic class the first shot is fired by work ing a lever, and the gun then continues to fire' until the ammunition is no longer supplied leaving the gunner' free to give his whole at tention to the aiming of the gun.
As to the relative merits of the two existing types of the automatic machine gun, expert opinion is divided, with perhaps a preponder ance in favor of the gun operated by the recoil, as a technically better design: the recoil has to be provided for in any case and is thus made to do necessary work. In this type the breech block is separated from the barrel and linked to it by multiplying cam levers. The recoil is taken up and partly stored by a powerful spring, which thus becomes the source of the other movements — which eject the empty shell, bring a new cartridge into position, in sert it in the firing chamber of the gun and explode it; this cycle continuing as long as there is a recoil to keep up the energy. In the gun operated by the gas pressure of the dis charge, a very small channel leads out of the interior of the gun barrel, and this is opened to the pressure of the explosion gases as the bullet passes. In some guns the gas is taken from very near the firing chamber ; in others it is taken near the muzzle just before the bullet leaves the gun. In both cases the channel leads to a piston which is thrust backward by the force of the explosion against a powerful coiled spring, which stores a part of the en ergy and returns the piston to its original po sition the instant the pressure is released by the bullet leaving the muzzle. In this type of gun the recoil is taken up by a buffer spring which does not do any work. The amount of force taken from the propelling gases of the explosion is so minute as to be negligible, espe cially when it is taken from close to the muzzle, when the bullet has acquired its full velocity.
It may be added that both methods work per fectly. The Maxim, Schwarzlose and Brown ing guns are operated by the recoil; the Colt, Hotchkiss and Lewis guns and the Browning rifle by explosion gases.
The great mechanical difficulty to be over come in a machine gun is the heating up of the barrel due to the rapidly recurring explosions. the cumulative effect being so to expand the barrel that the rifle grooves no longer grip the bullets, which fly out helter-skelter as from a smooth bore, falling to the ground after a very short flight. Two methods of keeping the gun cool while in action have been utilized: air cooling and water-cooling. In the former, thin "fins" of metal are affixed to the gun barrel so as to expose a comparatively very large sur face to the air which is expected to disperse the heat by radiation and convection. When the air is moist this device is quite successful, hut the drier the air the less it absorbs the heat and the quicker the gun will go out of commission. In practice it has been found that an air-cooled gun cannot be fired continuously for longer than three and one-half minutes, when it must be allowed to cool down, or an other barrel must be substituted. Tests have shown that 7,000 rounds fired without respite will destroy the gun barrel; but if the firing it stopped at the end of two minutes, and the gun cooled, the same barrel will fire 50,000 shots before a new one need he put in. On aircraft, however, the air-cooled gun never gets overheated owing to the swiftness of the pas sage through the air and the generally cooler air currents in which the aircraft move. Special mention may he made here of the Lewis gun, which is air-cooled in an unusual fashion. The aluminum "fins" in this gun lie lengthwise along the barrel and are about two inches in depth. The whole is encased in a shell of light steel; thus forming a jacket of 17 air channels open at both ends. When the gun is fired the ex plosion acts so as to draw air from the end next the breech through these air channels at a speed of 100 feet per second, proving a thor oughly effective cooling device. The Colt gun is so contrived that a puff of air is sent through the barrel after each shot.