I don't recall ever putting much more than ~100 BP rounds through a trapdoor in an afternoon, but bore fouling wasn't an issue. I don't recall ever trying that with a modern rifle--sounds like I might want to stick with an antique.
I visited 2 gun shops and 3 pawn shops today; no trapdoors. For that matter, no single shots or BP cartridge guns, and no primers or percussion caps. . . . One more pawn shop, and maybe a gun shop, to check before I go with Gbroker or see what Mr. Frasca might have. I've got 45-70 brass and dies, bullets (and a mould, but still packed up from the move), so I should be set once I find a rifle.