If you have the 28" twist barrel, that can stabilize full-bore lead projectiles (no sabot) of well over 400 grains. You have some experimentation to do.
Using PRB from my 48" twist barrel with 110 gr Pyrodex RS I was getting 3 to 5" (three shot) groups at 100 yards, which is almost as well as I can shoot any rifle using iron sights.
I then switched to real 2F BP, same charge volume, and I'm getting less velocity with larger groups and have to swab between shots or the tightly fit patched ball will stick on the way down. I could use a smaller ball with a thicker patch, fool around with the charge weight, or go to a conical and readjust the sights. I don't know, but it takes a lot of time to find that sweet load. Going back to RS and changing nothing else, I'd be OK with it.