Pitted bores on black powder guns are due to several reasons, not the least of which is dut to the fact that one of the byproducts of back power burning is Sulfuric acid. If it's not removed quickly it etches into the metal and produces a very accelerated corrosion rate.

Thats also why mufflers (on cars) rot out so quickly, burning gasoline also produces Sulfuric acid.

Ain't chemistry grand