The M1917 Revolver (formally United States Revolver, Caliber .45, M1917) was a U.S. six-shot revolver of .45 ACP caliber. It was adopted by the U.S. Army in 1917 to supplement the standard M1911 .45 ACP semi-automatic pistol during World War I. Afterwards, it was primarily used by secondary and non-deployed troops. There were two variations of the M1917, one from Colt and one from S&W.