Original Trapdoor? Don't shoot much jacketed in it as the steel was designed around a lead bullet and black powder. The rifling is designed around black powder and a lead bullet. It's safe with the low power loads only.
If you're going to play with black powder, stick with the REAL stuff as the modern subs are extremely corrosive and old soft steel won't fare well with a steady diet of the subs. And no, it's not elitist to advocate the real stuff over a sub. If you can't get the real stuff local, then get with some others who ship and split the hazmat fees. Otherwise, don't screw up a 95% condition piece of history.