The Colt M1917 and Smith & Wesson M1917 revolvers were adopted by the US military during World War 1 as a supplement to the M1911 Colt pistol, which could not be made in high enough numbers. The M1917 revolvers fired the .45 ACP round just like the M1911 Colt, and were loaded with moon clips since the .45 ACP is a rimless cartridge. The rounds could be loaded individually without the moon clip, but would have to be ejected using a pencil or cleaning rod.
The M1917 revolvers were also issued in World War 2, but in far lesser numbers. Today, they are very valuable collectors items. They are noted for their large and intimidating size in order to accommodate the wide .45 ACP round.