The .45 Auto Rim is a rimmed

Rim (firearms)

A rim is an external flange that is machined, cast, molded, stamped or pressed around the bottom of a firearms cartridge. The rim may serve a number of purposes, the most common being as the place for the extractor to engage...

 cartridge specifically designed to be fired in revolver

Revolver

A revolver is a repeating firearm that has a cylinder containing multiple chambers and at least one barrel for firing. The first revolver ever made was built by Elisha Collier in 1818. The percussion cap revolver was invented by Samuel Colt in 1836. This weapon became known as the Colt Paterson...

s originally chambered for the .45 ACP

.45 ACP

The .45 ACP , also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army in 1911.-Design and history:The U.S...

 cartridge. Remington

Remington Arms

Remington Arms Company, Inc. was founded in 1816 by Eliphalet Remington in Ilion, New York, as E. Remington and Sons. It is the oldest company in the United States which still makes its original product, and is the oldest continuously operating manufacturer in North America. It is the only U.S....

-Peters

Peters Cartridge Company

The Peters Cartridge Company was a company in Kings Mills, Ohio that specialized in gunpowder and ammunition production. Its historic buildings, built in 1916 at 1915 Grandin Road, were added to the National Register of Historic Places on October 10, 1985....

 developed the cartridge for use in the M1917 revolver

M1917 revolver

The M1917 Revolver 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...

, large amounts of which had become available as surplus following the end of the Great War. The M1917 had previously been used with half-moon clips that held three rounds of the rimless .45ACP. If half-moon or moon clips are not used with a rimless cartridge in a revolver, they must be ejected by hand with a rod or field-expedient tool like a pencil. In revolver cylinders not engineered to allow .45ACP to headspace properly, as in early production Colt M1917's, the cartridges could slip forward, stopping them from firing. Adding the rim solved both these issues.

Loads offered were similar to the standard military loads for the .45ACP, but with fully lead bullets rather than the harder alloys used for .45ACP. This was done to reduce barrel wear in the shallow rifled revolvers in which it was to be used. The .45AR case is stronger than the .45ACP case and has a slightly larger case capacity, allowing for increases in performance. It can deliver similar performance to standard pressure loadings in older, dimensionally larger, revolver cartridge designs like .45 Colt

.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...

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