You absolutely can fit a .45 Colt barrel to a .45 ACP gun if you're prepared to face down the shoulder to fit the barrel, and shorten the extractor. It's not a terribly difficult project if you have the tools. I installed an 8-3/8" barrel from a Model 25-5 on one of my 25-2s. I discovered it was a little heavier than I wanted, so I reinstalled the original 6-1/2" barrel. I gave the long barrel to Bob Perdue and he experimented with it on one of his revolvers.