The U.S. entered World War II with two submachine guns chambered for the .45 ACP round—the Thompson and the Reising—the latter of which was used primarily by the USMC, U.S. Navy and U.S. Coast Guard. The Reising was actually considered a short-barreled, pistol-caliber carbine more than a true SMG. When it went into action in the South Pacific, the Reising encountered various problems with reliability and parts interchangeability, and it was difficult to quickly disassemble and reassemble for maintenance.

The Thompson was a proven design that worked well, but it was heavy and expensive to produce, even in the simplified M1 and M1A1 versions. Circa 1939, the Thompson M1928A1 cost the government $209. For comparison, in 1939, a new Chevrolet would have cost $659. However, the M1A1 Thompson SMG designed for cheaper production costs could be produced for one-fourth the cost of the M1928A1. Despite the costs, over 1.5 million Thompson SMGs were produced in World War II.

As early as February 1941, the Small Arms Development Branch of the Ordnance Corps’ Technical Division issued a requirement for a “modern submachine gun.” The inexpensive and easy to produce British Sten SMG would influence the development of this “modern” SMG…