[sapper740]

Before I moved to Texas I had a substantial cartridge collection, one of which was a marker round for the 106mm Recoiless Rifle. It was an approximate .50 cal. Browning sized cartridge that created a flash and a puff of smoke when it hit. It was fired from a rifle that was clamped to the barrel of the 106 and had similar ballistics. It was a cheaper, and quieter method for gun crews to get some practice in. Apparently during WWII an inexpensive method to get some gunnery training was to clamp a solenoid fired weapon to the barrel or turret of a tank. here are two methods, one American, the other German.

first pic: M1928 on M4
second pic: M1928 on M4 another view
third pic: MG15 on barrel of Panzer