The M3 is a .45 caliber submachine gun developed in 1942 to be a replacement to the Thompson submachine gun. American made and adopted by the U.S Army in 1942, the M3 was cheaper to produce and lighter than the Thompson, but was less accurate. It received its nickname “Grease Gun” because of how similar it looked to the mechanics tool.
The M3A1 is a modernized variant of the M3 and contains several improvements including replacing the crank-type cocking lever with a recessed cocking slot. The modifications made provided for a more reliable, lighter weight, and easier to maintain submachine gun.