Green’s Dictionary of Slang

(US Und.) a machine gun.

1918		Eve. Star (Wash., DC) 15 Oct. 17/6: There fell to Scotty the chance to lie down with his beloved ‘sho-sho’ gun.
1927		(con. 1914–18) L. Nason Three Lights from a Match 95: He had only about forty men, three machine guns, no auto or ‘shosho’ rifles.
1929		W.R. Burnett Little Caesar (1932) 245: He ordered three sho-sho guns [...] These automatic rifles, as formidable as machine guns, were concealed in special cases.
1936		(con. 1917) Brooklyn Dly Eagle 11 Nov. 2/5: A whole battalion of machine and sho sho guns spitting.
1942		Detroit Free Press (MI) 29 Nov. 51/4: They had stored [...] many grease-packed Hotchkiss machine guns, ‘sho-sho’ tommy guns and some small arms.
1949		Monteleone Criminal Sl. (rev. edn) 207: sho-sho A machine gun.
1950		Goldin et al. DAUL 193/2: Sho-Sho. A machine gun.