Looking for second, third opinions

So I have an 1876 dated Colt model P, also called a single action army. I had a gunsmith who specializes in antiques swap out all the internals with a trigger job and rechamber from 455 mk 2 to 45acp, ergo, new cylinder as well. The barrel is not original, but an older rebarrel. This gun is a shooter grade, not collector grade as I don't have much of the original parts other than the frame. (Unless the previous internals are original, don't know). I doubt the previous 455 mk II cylinder is original just by how rare the SAA was in that caliber. But she's still priceless to me.

Anyway, my plan is to buy modern 45acp, and simply swap the smokeless powder for 20 grains BP. My question is does anyone forsee any issues using copper jacketed bullets in place of cast? I can easily use cast bullets but if I don't have to start casting and/or buying from retailers then I'd rather not. Factory loaded 45 ACP is much easier to find than primers are in Canada, which is why I'm thinking this route. Copper is still softer than steel, but I don't want to inadvertently put too much pressure on the iron framed old girl.

Thoughts? Good idea or stupid idea?