There are many underlever hammer guns that were originally proofed for cordite. Jack Lott (or one of the other older African hunters) used one such in .577 for Buffalo many years ago. It was written up in Gun Digest, Purple cover, I think (but may be wrong).
: It is a rare gun that was originally made for BP, but is re-proffed fro nitro. My good friend and double collector has one-such W.Richards 10 bore black powder double shotgun, refurbished by H&H, that was re-proofed for modern loads. It was virtually new, almost unfired when he got it, but was neglected and the stock needed re-finishing. He also wanted the original-type leather case for it.
: Today, one can find almost anything one wants. The trouble is affording them once found.