The big problem is crimping. The .45 ACP is .387" shorter than the Colt, so it's unlikely you could crimp a ACP in a Colt die. Most crimping dies work when they're about 1 turn off the shellholder. Next, the ACP is taper crimped and the Colt is roll crimped. If you have a Colt die that is short enough, crimping will require cases that are trimmed to the same length and a precise die setup. Most ACP bullets don't have a cannelure (crimp groove) so you've got no room for error.
Although the Colt's outside diameter is larger than the ACP, most sizing dies overdo it anyhow. Some Colt dies have a .454" expander, which is too big for the ACP. Pre-War revolvers use the larger bullet.
(Edited by Jack Monteith at 7:16 pm on Jan. 17, 2002)
All the rimless autoloaders headspace on the casemouth. A roll crimp ruins this relationship.
I crimp my cast SWCs a bit out and headspace on the bullet in my bullseye gun -- this gives a nice tight headspace. Might not be appropriate in a defense gun.
Doing it my way should be tuned to the individual narrel, chamber tolerances might not let my ammo shoot properly in a different pistol.
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