When was the gun invented that chambered the same .45 round as the M3 submachine gun?
M3 submachine gun:  The M3 was chambered for the same .45 round fired by the Thompson submachine gun, but was cheaper to produce, and lighter, although, contrary to popular belief, it was far less accurate. Thompson submachine gun: The Thompson submachine gun is an American submachine gun, invented by John T. Thompson in 1918, that became infamous during the Prohibition era.
1918