what's the twist rate or the rifling? If it's 1 in 78"(which is standard) it should do OK with a hollow based conical in the 400 grain range. Keep the bullets as close to pure lead as you can. Use a well lubed felt wad between the bullet and powder to help keep a good seal at the bullet base. Start with 70gr Fg or FFg black and work up from there until you find that perfect load.

You can make your own lube at home, or buy the T/C bore butter kind of stuff in a tube.

You might also find that a .570" cloth patched round ball might shoot well to.