M3 submachine-guns for sale. The M3 was an American .45-caliber submachine gun that entered U.S. Army service on Dec. 12, 1942, as the United States Submachine Gun, Cal. .45, M3 and began to replace the .45-caliber Thompson series submachine guns: the M1928A1, M1 and M1A1 that were slowly being withdrawn from use. The M3 was designed from the outset as a more cost-effective alternative to the Thompson, optimized for mass production. The M3 is commonly referred to as the "grease gun", owing to its visual similarity to the common mechanic's tool.