crowkiller
First thing you want to do is slug the bore. Easiest way is to use a soft lead sinker and drive it down the bore with a wooden dowel. On my 38-55 target model, I used the tapered trianular/bell shaped sinker. Before I used the egg shapped ones and they were a lot harder to push through.
With the bore being in the neighborhood of .458 you and the slug to start off at .460-.465. Any more is very hard to push, any less might not get the true bore size.
Once done, mic the slug and fing the bore and land dia. Your bullets you use should be at or larger than bore dia. If it's ,458 bore then bullets from .458-.460 should be ideal. I'd suggest a 1-20 or softer lead bullet for BP usage. Don't forget, NO EMPTY SPACE between BP and the bullet, or base wads, if you use them.
LOTS more from there on powders, wads, lube etc. Look at the forum eskimo pointed you to and at
www.shilohrifle.com They have user forums there with YEARS of BPCR loading.
Good luck.
