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by Max R. Popenker
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M3 and M3A1 submachine gun / "Grease gun" (USA)

M3 submachine gun, left side view, shoulder stock fully retracted

M3 submachine gun, partially disassembled. Note dual return springs and a separate cocking mechanism housing

M3A1 submachine gun. Note enlarged dust cover, absence of a cocking handle and optional flash hider

rare version of M3A1 submachine gun, fitted with integral silencer

Caliber: .45ACP (also 9x19mm Luger/Para in M3A1)
Weight: 3.7 kg
Length (stock closed/open): 570 / 745 mm
Barrel length: 203 mm
Rate of fire: ca. 450 rounds per minute
Magazine capacity: 30 rounds
Effective range: 50 meters

The M3 submachine gun, also known as Grease gun, was developed as a cheaper war-time alternative to famous Thompson M1 and M1928 submachine guns. The basic requirements were set by US Ordnance Corps in February, 1941. The George Hyde and Frederick Sampson, working together at Inland Division of general Motors Corp developed a prototype, which was designated as T20. This was a very simple weapon, made mostly from steel stampings. In November 1942 T20 was tested against several other prototypes, and was found superior to all other contestants. Late in 1942 the T20 was recommended for adoption, which followed in 1943. Combat use of a newly adopted M3 submachine gun showed some problematic points in design, most notably in cocking mechanism. The problems were solved by elimination of this unit, which was done in 1944. The further simplified weapon received designation M3A1, and served with US Armed forces through the later part of WW2, Korean and Vietnam wars. M3A1 was issued to US tank crews up until 1980s.

M3 submachine gun is full-automatic only, blowback operated firearm that fired from open bolt. The receiver is made from steel stampings. M3 featured spring-loaded ejection port cover (which also also act as safety, locking the bolt when it is closed) and crank-type bolt retracting (cocking) handle at the right side of the receiver. In M3A1 the designers removed cocking handle assembly (which was source of malfunctions) and replaced it with simple finger hole in the bolt body, accessible through enlarged ejection window. Also, M3A1's could be converted for 9x19mm Luger ammunition by replacing the barrel, bolt, and installing the magazine adaptor to use British STEN magazines. 
The retractable stock, made from steel wire, could be used as cleaning rod (when detached), and it also featured a magazine loading tool. The hollow grip of the gun contained a small oilier, which was necessary as the all-steel gun rusted easily in wet climate.
A special version of the M3A1 was produced for clandestine operations; it featured long, integral silencer.

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