We cut the 45 Colt cases the exact overall length as the ACP and AR; just long enough for the case mouth to headspace on the ACP shoulder in the chamber. The Colt rim sits .025" off the cyl face and sticks out of the chamber just like an ACP case does, with or without a clip. The primer is the same distance from the recoil shield as the ACP or AR primer. Proper headspace is maintained.

The AR headspaces on it's thicker rim (and its case mouth).

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Jim
S&WCA #819