So long as it doesn't take fixed cartridges the BATF doesn't care a bit. Firearms built before 1899, or replicas thereof that take obsolete unavailable ammunition, or that do not take a fixed cartridge are not technically firearms. They are relics, antiques, whatever. You can do almost anything to them. They are still dangerous weapons in most places, so you would need a permit to carry it concealed, but you could certainly hunt with one with nothing more than a hunting license.