Subcaliber firing was used by the U.S. Army to develop a tank gunner’s accuracy, speed, and confidence without the costs and disturbance of firing the main armament. In general, the coaxial machine gun was used for the subcaliber training for tank crews. However for fire adjustment training, a standard submachine gun could be mounted outside the tank. Until a standard mount could be developed, the Tank Gunnery manual recommended the following mount for the 75-mm gun on the M4. For firing, the submachine gun was connected to the firing solenoid from the coaxial machine gun. Source: Tank Gunnery, War Department Field Manual FM 17-12, July 10, 1944.
