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aerial combat do not allow many shots to be fired at a time, aircraft guns do not suffer from overheating as ground guns do. In addition, the rush of the aeroplane through the atmosphere is very effective in cooling the guns. For this reason no cooling devices are employed in aircraft guns. In the case of the Lewis guns the radiators are removed; and when the Vickers or other water-jacketed guns are used the water-jackets are cut open to allow a free circulation of air and to reduce weight. On account of the very high speed of modern aeroplanes, the relative n otion of passing machines is great enough to leave a very large interval between machine-gun bullets fired from one aeroplane at another. Inordertoreducethis interval and increase the chances of a hit, most aircraft machine-guns are arranged to work at a higher rate of fire than ground guns are. The Browning aircraft gun fires over I, coo shots per minute.

Large-Calibre Machine-Guns. The desire for an increased incen- diary effect against balloons led to the adoption by the Allied armies of aircraft machine-guns of a larger calibre than that of the standard service cartridge. The first guns of this type were Vickers guns chambered for the old ll-mm. (-44 in.) French cartridge. These cartridges were loaded with a tracer bullet in which the burning of the tracer composition was sufficiently intense to give an incendiary effect on impact with a balloon. A large number of the n-mm. Vickers guns were used, but, owing to the fact that the velocity of the cartridge was low (1,300 f. s.), a more powerful cartridge of about this calibre was considered desirable. The result was that large- calibre (-45 to -50 in.) machine-guns, intended to give a velocity of about 2,500 f. s., were designed in both America and Great Britain during the latter stages of the World War. These designs, however, were not completed in time for any of the guns to be placed in service before the Armistice.

Tank Machine-Guns. The introduction of the tank as a weapon of warfare was followed by the adoption of the machine-gun as part of its armament. This use of the machine-gun, like its use on air- craft, is so different from its normal employment as to call for a spe- cial type of gun or for a special adaptation of existing types. By virtue of its heavy armour protection, the tank can approach very close to hostile positions, and is consequently the target of an extreme concentration of rifle and machine-gun fire. For this reason, water-cooled machine-guns are unsuitable for the armament of tanks, because the water-jacket, which must necessarily project out through the armour, would soon be riddled. For the same reason it is desirable to have for a tank machine-gun a weapon with a barrel heavy enough to withstand rifle fire. For this purpose the French used the Hotchkiss, and the British used the portable Hotch- kiss, or Benet-Mercifi, which was admirably suited for this use. The first tanks built in America were armed with the Marlin tank-gun, which was the Marlin aircraft-gun with an aluminium radiator added. Later a special Browning-type gun was designed for use in tanks. This weapon, which is called the Browning tank-gun, is similar in its mechanism to the Browning machine-gun, with the difference that the water-jacket is omitted and the barrel is made very heavy, and is only 1 8 in. long instead of 24. There is a special latch provided on this gun to hold the mechanism open when the gun is not firing, and thus prevent the heat of the barrel from exploding the cartridge.

FIG. i. U.S. Army Tripod.

Machine-Gun Mountings. The universal method of firing ground- type machine-guns is from a tripod, which allows the gun either to be controlled lor elevation by means of handwheels and clamps or to be swung free. The British Mark IV. tripod is an excellent example (see 17.246). This very simple and rugged device remained standard in the British army during the World War, and was used extensively by the American army. The three legs are each adjust- able by means of clamps which allow their relative positions to be changed to compensate for inequalities of the ground, or to alter the

height of the gun. Changes in elevation are obtained by means of an elevating screw and hand-wheel. There is a friction clamp for deflection. Small changes in azimuth are obtained by setting up the deflection clamp lightly, and then " tapping " the gun with the hund on one side or the other until the proper direction is obtained. It will be noted that by throwing off the deflection clamp this gun can be swung freely from side to side, which is important in meeting a rush at close quarters, but there is no hand- wheel or other device for setting off a given deflection from a known direction, which is often very

FIG. 2. German Machine-Gun Mounting.

desirable in indirect fire. It will also be noted that there is no satisfactory way of allowing the gun to be swung freely in elevation, which is important in shooting at aircraft. When this becomes necessary, the elevation pin must be pulled out, and the gun must be reversed in its cradle. These disadvantages have been overcome, though at the expense of a considerable amount of mechanical complication, in the U.S. army tripods, models of 1917 and of 1918 (see fig. I ), which have a slow-motion hand-wheel and index, and a throw-off for both elevation and deflection. These devices allow the gunner to make adjustments of known amounts in indirect fire, or to swing the gun freely in any direction when this becomes desirable. The French tripods have a hand-wheel for elevating, and a clamp for traversing. The elevating screw is attached to the gun by a snap-

FlG. 3. Vickers Machine-Gun.

catch which can be quickly released to allow free elevation. Stops, which can be set to limit the traverse to any desired amount, are provided. The legs are provided with a quick adjustment to allow the use of either a high or low gun position. The Germans used tripods to some extent, but placed their main reliance on a distinctive style of mount which can best be described by refer- ring to the illustration (fig. 2). This device provides both for mount- ing the gun and for transporting it. The gun is fastened into the mount by means of trunnions on the water-jacket, which fit into gimbals so that the gun may be pointed in any direction. There is a slow-motion hand-wheel for elevating the gun, and a throw-off which allows it to be swung free. The deflection is controlled by means of a friction clamp. The mount is hinged in the middle so as to be adjustable for height. When it is desired to transport the gun, the