.45 ACP
The .45 ACP (Automatic Colt Pistol), also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning
John Browning
John Moses Browning , born in Ogden, Utah, was an American firearms designer who developed many varieties of military and civilian firearms, cartridges, and gun mechanisms, many of which are still in use around the world...

 in 1904, for use in his prototype Colt
Colt's Manufacturing Company
Colt's Manufacturing Company is a United States firearms manufacturer, whose first predecessor corporation was founded in 1836 by Sam Colt. Colt is best known for the engineering, production, and marketing of firearms over the later half of the 19th and the 20th century...

 semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

 in 1911.
The U.S. Cavalry
United States Cavalry
The United States Cavalry, or U.S. Cavalry, is the designation of the mounted force of the United States Army. The role of the U.S. Cavalry is reconnaissance, security and mounted assault. Cavalry has served as a part of the Army forces in every war in which the United States has participated...

 had been buying and testing various handguns in the late 1890s and early 20th century. The .45
.45 Colt
The .45 Colt cartridge is a handgun cartridge dating to 1872. It began as a black powder revolver round developed for the Colt Single Action Army revolver, but is offered as a magnum level handgun hunting round in modern usage. This cartridge was adopted by the U.S. Army in 1873 and served as the...

 Colt Single Action Army
Colt Single Action Army
The Colt Single Action Army is a single action revolver with a revolving cylinder holding six metallic cartridges. It was designed for the U.S...

 had largely been replaced, even by some double action versions of the same.