tommy gun 
noun informal
A type of sub-machine gun.Meanwhile our heroes have a couple of handguns and a tommy gun....
- Capone's usual partner was Jack McGurn, a sleek, handsome virtuoso of the tommy gun, architect of the St. Valentine's Day shootings.
- In one of the film's most famous scenes, Camonte secures a tommy gun from a now dead, would-be assassin and, with the joy and fascination of a child at Christmas, tries out his new toy.
Origin
1920s: contraction of Thompson gun, named by its designer after John T. Thompson (1860–1940), the American army officer who conceived the idea for it.