The M3 SMG reference article from the English Wikipedia on 24-Apr-2004
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M3 SMG

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. It had a length of 745mm, but due to its retracting stock, could be shortened to 570mm. The M3A1 was used during World War II and Korea. 679,200 were produced.
A photo of M3A1 with stock retracted
M3A1 with stock retracted
image from Modern Firearms