You can basically shot black powder cartridge loads in about any modern firearm made(not smart in semi-automatics). You just have to make sure you do a thorough cleaning job.

I am sure there will be more regulations than just allowing any BP cartridge.

A good single shot rifle would make a fine choice. There are alot of single shots made specifically for BP cartridges. Alot of the Sharps rifles(and reproductions) do very well with BP. Tom.