MODERN MACHINE GUNS The machine gun is an automatic small-arm weapon, usually provided with a fixed mounting and capable of sustained accurate fire. A heavy machine gun does not differ materially from an ordinary machine gun in design, but as its calibre may be from 0.5 to 0.9 in., it is necessarily of a heavier and stronger make. The weight of a machine gun, however, if in the hands of infantry or cavalry, is limited by the consideration that, by whatever means it may be transported, it must in the last resort be "man-handled" into position often over considerable distances and under difficult conditions. On the other hand, lightness and portability are desirable only in so far as they are consistent with strength, for to a large extent it is on the strength and, therefore, weight of the mechanism that the ability of a gun to sustain fire depends.
Mounting.—The fixed mounting with which the machine gun is provided increases the accuracy of all ordinary shooting, and thus enables the gun to be used either for indirect fire or for fire over the heads of friendly troops, when supporting an attack or covering a retirement. To support rigidly a machine gun weighing from 4o to 5o lb., the mounting must be strong and fairly heavy, otherwise the combination will be top heavy and unsteady, and the shooting in consequence will be inaccurate. The mounting must permit of the gun being traversed through as wide an arc as possible, and also permit alterations to be made in elevation.
Machine guns for ground use normally have a rate of fire of about 450-500 rounds per minute, but, owing to the necessary replenishment of empty belts or magazines, etc., the number of rounds actually fired per minute is much less. In aircraft a higher rate of fire is desirable and is usually obtained.
Cooling Systems.—For sustained fire an efficient cooling sys tem for the barrel is essential. The higher the temperature of the barrel, the less its resistance to wear. There are two systems in use, namely, radiation in air and water-cooling. Heavy barrels with radiating rings are sometimes employed for ground use (e.g., the French Hotchkiss). The water-cooling system is the most common. The barrel for nearly the whole of its length, is sur rounded by water contained in a casing which, with recoiling bar rels, is fitted with glands to prevent leakage. Inasmuch as pro longed rapid fire causes the water to boil, a steam escape is necessary. Accordingly a flexible tube is usually attached to the
steam escape whereby the steam is led to a portable condensing chamber, and converted back into water for use again in the gun. The water increases the weight of the gun by about 9 lb., and for this reason light machine guns rely on radiation for their cooling.
Mechanical Safety.—All automatic weapons are required to be "mechanically safe," that is, the design must be such that the gun cannot be fired unless the action is fully closed and the cartridge properly supported. Even then, it should only be able to fire by means of the trigger mechanism, which itself must be proof against heavy jarring. Safety catch ("applied safety") devices should operate directly on the firing pin or striker, and should preclude the possibility of this ever being able to reach the cartridge by accident.
For the automatic operation of machine guns power may be obtained in three distinct ways, viz., from the gases generated by the explosion of the charge, from the recoil of the barrel, and from the backward thrust of the cartridge on explosion. In some types of gun two of these principles of operation are corn bined. The French Hotchkiss gun is gas operated, on principles which have been explained in connection with light machine guns.
Barrel Recoil Only.—In the U.S.A. Browning water-cooled gun, when a round is fired, the barrel, barrel extension, and breech block move backwards locked together fors inch. A locking block is forced down clear of the breech-block, and the latter travelling further to the rear compresses the mainspring. A lever working in a slot on the breech-block operates a slide, which, by means of pawls, feeds the belt of cartridges into the gun. On the breech block is carried the extractor which takes a round from the belt as the breech-block goes backwards, and places it in guides on the front of the block. When the breech-block is driven forward by the mainspring, the lever and slide move to grip a new round, and the round already in position is placed in the chamber. The trigger action inside the breech-block is cocked during the back ward movement. This action is so designed that the trigger is unable to operate the sear unless the breech-block is fully forward and therefore locked. The gun is thus mechanically safe.