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The M3A1 is a .45 ACP submachine gun developed by the United States Army during World War II. It was designed as a cheaper and faster-to-produce alternative to the Thompson submachine gun. The weapon earned the nickname Grease Gun due to its resemblance to the automotive tool used for applying grease. The M3A1 simplified the original M3 design by eliminating the cocking handle and using a finger hole in the bolt instead. It remains one of the most cost-effective submachine gun designs ever put into mass production.