Thanks for everyone's responses. In wake of this information, I recently purchased an (approximate date) 1875 .500 bpe jones underlever double that was bored to 20 bore (I assume for buck and ball loads for leapord). The action to barrel fit is tight with absolutely no play, and both locks function relatively well (in dire need of cleaning). One of the barrels is dented, and I had originally planned to fix the dent and sleeve the barrels back to .500 bpe. Before I move forward with this process could anyone tell me if this action is strong enough to be chambered in .375 H&H? I would install a new set of firing pins/bushings, and cut/thread the monoblock for two new barrels + machine a new extractor etc.