Redhawk1
I'm surprised, but trust your experience. I have found that black powder performed better than pyrodex or 777 in my cartridges (43 Span, 43 Mauser, 45-70 and 50-70). The trapdoor is the only one that's a PITA to clean. I don't like the rod coming in from the muzzle. Then, there's the price differential.
It's interesting that there's such a difference between ML vs cartridges loaded using black powder. Perhaps the cartridge generates higher pressures?
jeff