I would also look at the New England Firearms system which can be available not only in BP .50 cal but in different barrel and caliber configurations for shooting different game from shotguns to your most popular centerfire calibers.

I bought one with 6 different barrels ranging from .50 cal. 223, 270, 22 Hornet, 20 gauge bull barrel slug gun, 12 ga. slug barrel.

It's an extremely well put together system, not that expensive, easy to clean, ACCURATE, stable and well worth the money.