I cast my own bullets and use the RCBS 400 gr., and the NEI 216A, a 433 gr. Postell-style bullet designed especially for long range by Dick Gunn. If I was starting over again, I would select a 300 - 350 gr mould and use it for the shorter ranges (up to maybe 300 yd, or for the chickens and pigs), and then use the 216A for everything else.
I have shot a little silhouette, and will be doing more, but mostly I shoot "gong" matches. The Browning has a Badger barrel, which is one of the best, and is a very accurate rifle. The only negative is that the trigger needs some work. Dale McGee told of a simple way to fix it in the April 2000 (?) issue of Single Shot Exchange. He will e-mail you the article - but he just left for a 1 month vacation!
My gun will shoot the RCBS bullet into about 1.5" at 100 yd., and the NEI will do a little better than that. I use Goex 3F in mine, for a MV of 1225 fps (NEI) and 1250 fps (RCBS).
What you really need to do is go over to the "Blackpowder Cartridge" board on shooterstalk.com. That is, by far, the best board on the web on this subject! (Dale McGee posts there under the name "Dale53").