[3] Two issues related to the use of .45 ACP ammunition in the M1917 revolver led to the development of the .45 Auto Rim.

The M1917 had previously been used with half-moon clips that held three rounds of .45 ACP, a rimless cartridge.

In revolver cylinders not engineered to allow .45 ACP to headspace properly, as in early production Colt M1917s, the cartridges could slip forward, stopping them from firing.

Adding a rim to the .45 ACP cartridge solved both these issues.

[4] Loads offered were similar to the standard military loads for the .45 ACP, but with fully lead bullets rather than the full metal jacket bullets used for .45 ACP.