The Colt Single Action Army handgun (also known as the Colt Peacemaker, Single Action Army, Colt .45 and sometimes as The Equalizer) is a single action revolver with a revolving cylinder holding six rounds. It was designed for the US government service revolver trials of 1873 by Colt"s Manufacturing Company and adopted as the standard military service revolver.

The .45 Colt cartridge was of center fire design containing charges of up to 40 grains (2.6 g) of fine grained black powder and a 255-grain (16.5 g) blunt round nosed bullet. Relative to period cartridges and most later handgun rounds, it was quite powerful in its full loading.

The Colt Single Action Army handgun replaced the Colt 1860 Army Percussion revolver and remained the primary US Military sidearm until 1892 when it was replaced by an enclosed frame Colt double action revolver. By 1875, 15,000 units chambered for the .45 Colt cartridge had entered service along with an additional 1863 chambered for the .44 Henry rimfire cartridge (Wilson 1985.)

By the mid 1870s, the Army had purchased a significant number of Smith and Wesson revolvers chambering a shorter .45 round. Logistical problems arose because the ammunition was not interchangeable. The Colt revolvers would accept the shorter round but not vice versa. For a time, the Government stopped orders for the longer Colt cartridge and used the Smith and Wesson round exclusively.