The rate of fire in the air can not be made too swift. Suppose an airplane were flying past a long, stationary target, such as a billboard, at the relatively slow speed of 100 miles an hour. Assume on this plane a flexible machine gun aimed at the billboard at right angles to the line of flight. If this is a fast machine gun, it may shoot 880 times a minute, at which rate the shots will come so fast that the explosions will merge into a continuous roar. Yet the bullets fired at such a rate from a machine moving at even such low speed will be spaced out along the billboard at intervals of 10 feet. But most of the fighting planes traveled much faster than 100 miles per hour. Thus it is entirely possible for two antagonists in the air to aim with complete accuracy at each other and both to pass unscathed through the lines of fire. The faster, therefore, the aircraft gun fired, the better the chances of bringing down the enemy plane.
The Lewis gun, invented by Col. Lewis, of the United States Army, was the weapon most generally used by the allies as the flexible gun for their airplanes, operated on a universal mount which permitted it to be pointed in any direction. The Lewis aircraft gun was the ground gun modified principally by stripping it of the cooling radiator and by the addition of a gas check to reduce the recoil. The Lewis was fed by a drum magazine, a more desirable feed for flexible guns than any belt system. The German flexible gun, the Parabellum, had the unsatisfactory belt feed.
The Vickers gun was the only successful weapon of the fixed type developed in the war before we became a belligerent. We were manufacturing Vickers guns in the United States prior to April, 1917; but when the Signal Corps faced the machine-gun problem, in September, 1917, it found that the Infantry branches of the Army had contracted for the entire Vickers production in this country.
Accordingly, the equipment division of the Signal Corps, in the face of marked opposition, took up the development of the Marlin gun as an aircraft gun of the fixed type. This gun, however, proved to be extraordinarily successful and was regarded by our Flying Service and by the aviators of the allies to be the equal of the Vickers in efficiency. Because of this development, when there came the need of tank guns, in June, 1918, the Aircraft Board, which had succeeded the Signal Corps as the director of aerial activities, was able to supply 7,220 Marlin machine guns within two weeks for this purpose.
The first order for Marlin guns was placed on September 25, 1917; and over 37,500 of them had been produced before December, 1918. The Marlin-Rockwell factory began producing 2,000 guns per month in January, 1918, and increased this rapidly until as many as 7,000 guns were built in one month. The Marlin gun shoots at the rate of 600 to 650 shots per minute and is fed by a belt of the disintegrating metal-link type.
As to Lewis guns, which we adopted as our flexible weapon, more than 35,000 of them were delivered to the Air Service up to December, 1918. In February, 1918, the Savage Arms Corporation built 1,500 of them, increasing their monthly deliveries until in the month of October, 1918, they turned out 5,448 of these weapons. The Lewis gun which the British had been using carried 47 cartridges in its magazine. A notable accomplishment of the manufacture of Lewis guns for our use was to increase the capacity of the magazine to hold 97 cartridges.
In our De Haviland-4 planes we installed two Marlin fixed guns, each firing at the rate of 650 shots per minute, equipping the weapons with Constantinisco controls to give the plane a maximum fire of 1,300 shots per minute through the blades of a propeller whirling at a rate as high as 1,600 revolutions per minute. Four fixed guns have also been successfully fitted to one plane and timed so that none of the bullets struck the propeller blades.
At the time the armistice was signed the rate of production of special aircraft ammunition, a classification including tracer bullets, incendiary bullets, and armor-piercing bullets, exceeded 10,000,000 rounds per month.
The original estimate for the quantity of ammunition our Flying Service should have was later greatly increased because the squadrons at the front began installing as many as four guns on a single observation plane.
Although different aviators had their own notions about the loading of ammunition belts, certain sequences in the use of the three types of special ammunition were usually observed. First usually came the tracer cartridge, which assists the gunner in directing his aim; then two or three armor-piercing cartridges, relied upon to injure the hostile engine or tap the gasoline tank; and finally one or two incendiary cartridges to ignite the enemy's gasoline as it escaped, sending him down in flames. Such a sequence would be repeated throughout the ammunition belt or magazine container.