Thompson submachine gun M1921
.45 ACP
Developed too late to be used in World War 1, the "Chicago Typewriter" was made famous by movies about 1920s-1930s gangsters, even though not many gangsters used them.
Auto-ordnance hoped to sell large quantities and had the parts mass produced, but ended up stock piling most of them for over a decade as they slowly made sales to the US Postal Service and some Police and military forces. The FBI used Tommy Guns from 1933 right up until the 1960s.