Adding to what Jimmy S said, there are also cappers for 209 shotgun primers, which are the only recommended primers for use with pellets. Most modern in-lines come set up for 209s, or with a replacement breech plug.

Cleaning is essential. Don' t let your in-line sit overnight after firing it, or you may quickly run into corrosion problems. Black powder (and the substitutes) is very corrosive, and in-line barrels are generally not chrome-plated inside.

BP can be a pain in the behind, but it does extend the season