The '75 is a great gun, but it doesn't handle fouling well. When I shot mine with BP, I would put some extra BP lube over every other chamber mouth like one would with a muzzleloading Revolver. This, combined with Big Lube bullets, gave decent performance. Can't speak for APP. Mine didn't like Alliant Black MZ, which I've heard is similar.
Posting from my phone, so I might have some typos. My apologies.