Quigley, I shoot pure lead (and blackpowder) in my conversions. In the .38, which is a 2nd. Generation 1861 Colt converted by R&D, it has the original sized barrel, and I shoot pure lead, hollow based bullets in it (.38 Long Colt). I also have a Uberti 1851, but with the larger bore size.....like the 1861, it performs very well with blackpowder and soft lead.
In my .44s (an 1860 and an 1872 Open Top), the bores are .429, and I shoot .430 diameter bullets that are cast pure. I DID try harder cast bullets, and just didn't get results that I was happy with.
I also have a pair of Dragoon Conversions, in .45 S&W caliber. Same thing, I shoot pure lead in them.