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.

However for fire adjustment training, a standard submachine gun was 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. 

July 10, 1944

Gunnery Training

Mounted on a Sherman M4, the T34 rocket launcher had 60 tubes that fired a 4.5&quot; rocket to a maximum effective range of 3,900 yards. The T34 was only made in small numbers and was assigned to the 2nd Armored division in France. A problem with the T34 was that the tubes started to break up after only 4 salvoes and tube life was only 15 - 20 salvoes. The rockets could be fired singley or in salvo. All 60 rockets could be fired in 30 seconds, and each rocket hit with the impact of a 105 mm round. The elevation of the Calliope worked off the elevation of the main gun, and as long as the Calliope was attached the main gun could not be used. However the Calliope could be jettisoned hydraulically in a matter of seconds.

Calliope - Sherman with T-34 rocket launcher system

M4 Sherman medium tank, armed with a low-velocity 75mm main gun. The tank sports extra welded armor on its hull.  