303:
There are a number of Colt .45-calibre handgun cartridges, but the two common ones are the .45 Colt and the .45 ACP (Automatic Colt Pistol).

The .45 Colt is a large-case rimmed cartridge which originated in the revolvers and lever-rifles of the American West.

There is a tendency to refer to this cartridge as the .45 Long Colt these days to avoid confusion with the much shorter rimless .45 ACP.

The .45 ACP is mainly chambered in automatic pistols like the 1911, so might be referred to by some as the .45 auto. A few .45 or .455 revolvers can chamber this round with the aid of a circular clip.

The term '.45 auto' should probably be discouraged as it could result in confusion with the .45 Auto Rim, a semi-rimmed pistol cartridge not commonly encountered today.