The .45 Auto Rim, a.k.a. 11.5x23R is a rimmed cartridge specifically designed to be fired in revolvers originally chambered for the .45 ACP cartridge.
The Peters Cartridge Company developed the cartridge in 1920 for use in the M1917 revolver, large numbers of which had become available as surplus following the end of World War I.
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 full metal jacket bullets 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.
The round is currently still in production by Corbon in their DPX and Performance Match lines of ammunition and is also manufactured by Georgia Arms.