and rimless cases do not work terribly well.
Straight rimless (or essentially straight) were never used mostly due to issues with headspacing. Bottle neck rimless rifle rounds work JUST FINE.
.250-3000, .243. .300 Savage, .308 Win, and .35 Rem being the most common. There are a couple guns that even took the .30-06
Probably have to modify the extractor.
Going from .45Colt to .45ACP? probably, since the .45acp is spec'd at .480" rim (and that would be a max dimension, most brass will be a bit less than that) and the .45 Colt is spec'd at .512".
Next question, does the bolt face itself have the clearance needed so a new/modified extractor can reach "in" as much as it needs to to grab and hold .45acp cases??
or could it be done with a different design of extractor hook? I don't know, never considered the idea before...
The Savage 99 and Browning BLR are already well set up for the .45acp size case head and body diameter (same as the .308) but you'd have a couple inches of "empty" space the ACP would have to move through to get to the chamber, so some kind of spacer system would need to be designed.
Plus, neither of those is a cheap rifle to experiment on, and the Savage is out of production, anyway...
And, if you could get it to work, you'd have only a fraction of the ammo capacity of a tube magazine gun. .45ACP should work ok in a tube mag, the RN is fairly blunt. Pointed 9mm (FMJ) would be a BAD idea in a tube magazine.
The most practical alternative to semi auto PCC's is simply the existing lever action PCCs, accepting their chamberings as they are.