I've seen barrels and breeches where Pyrodex fouling has, over time and shooting, eaten a hole through 1/4" of steel. It seems to have an affinity for the iron molecules in the steel, sort of disolving them, when looked at under good magnification - sharp edged craters where the material is simply missing, as if disolved. I've been told by a chemical engineer, that after starting, the holes actually deepen from the firing pulse of sucessive shots.

This is my opinion, of course.

I'd use smokeless loads or real black powder, wihch cleans up easily & completely.