violin case to carry a submachine gun - the St. Valentines Day massacre featured this ruse

Actually, the practice long predates 1920's Chicago, and traces back to England in the middle ages. The Saxons, adept at playing the Angles, adopted the ruse of thus concealing their primitive weapons, in order to take the neighboring tribes unaware. This practice is de-Picted in contemporaneous writings, whence originates the longstanding tradition of Saxon Violins in the popular media.