Green’s Dictionary of Slang

powder wagon n.

[like SE powder-wagon it carries an explosive charge]

(US Und.) a sawn-off shotgun.

1929

C.G. Givens ‘Chatter of Guns’ in Sat. Eve. Post 13 Apr.; list extracted in AS VI:2 (1930) 134: powder-wagon, n. Sawed-off shotgun.