From a school tour of the FBI that I did in ~1970, a bunch of gangsters acquired a 20mm anti-tank gun to use against armored cars in the 1930s. They were caught before they could use it.
Thunderbolt & Lightfoot.
http://www.casttv.com/video/zj9f5fp/thunderbolt-and-lightfoot-michael-cimino-1974-theatrical-trailer-video