US M3

Submachine Gun, Cal. .45, M3/M3A1 (1942) was nicknamed the "Grease Gun" because of its appearance. The M3 was a blow back submachine gun fielded in December 1942 by the United States Army during World War II. It could only be fired on fully-automatic. It used a .45 caliber cartridge in 30-round magazines. Its rate of fire was 350-450 rounds per minute. The M3A1 was used during World War II and Korea. Following World War II, the role of submachine guns was greatly diminished with the introduction of assault rifles and light portable machine guns. Submachine guns are still used by special forces, air crews, armored vehicles, counter-terrorist units, and Naval personnel. 679,200 were produced.

M3A1 with stock retracted
image from Modern Firearms

M3 (1942) was designed specifically to simplify production by making use of stamped metal parts. A number of deficiencies were found during the initial two years of use. These deficiencies were corrected in the model M3A1.
M3A1 (1944) had a larger ejection port and a stronger cover spring. It was also designed so it could fire 9mm Parabellum cartridges by changing the barrel and bolt and adding an adapter to the magazine. Following World War II, a curved barrel was made for use with the M3A1. A flash hider was developed for use with both the M3 and M3A1.