I shoot a lot of BP and subs (usually APP), as I shoot Cowboy Action and shoot BP in pistol, rifle and shotgun. One thing I have found in shooting BP in shotgun is that because of the really hot combustion, it melts plastic wads and coats the barrel with plastic. I only use fiber wads now. I noticed that same thing with Powerbelts. They have the green plastic cup and I got lots of green plastic residue in the barrel. Accuracy was not good after a couple of shots. Cleaning is harder than just cleaning the normal BP residue.
I use nothing but Thors now.