Green’s Dictionary of Slang

to blast with a sub-machine gun.

1960		E. De Roo Big Rumble 68: He got his squashed foot from Big Tony’s father, Chimp, who was tommy-gunned in Chicago.
1987		(con. c.1967) J. Ferrandino Firefight 10: Staring at the screen as a GI tommygunned two dozen Nazis.