beach,

For the sake of clarity, pitting can occur on both the inside and the outside of the barrel, it is basically very confined corrosion. Now, since you are posting this in the blackpowder forum I imagine you are wondering about pitting inside the barrel. Pitting is caused by improper (or usually no) cleaning. What happens is the powder byproducts attract moisture and the salts remaining in the residue start eating away at the metal. If you clean and oil your bore after every session most likely you will not ever have a pitting problem.

Now, lets say you get a gun that has a lot of pitting when you look down the bore, what can you do? First, it may shoot just fine with the pits in there. The main trouble I have had with pits is they can hang up the patching material and make it inconsistent to seat the bullet uniformly, but I've only had 1 gun give me problems so I'm no expert.

If you have a barrel with pits, the bore can usually be polished pretty good with steel wool wrapped around the cleaning rod and use something like Flitz or Simichrome metal polish.

Hope this helps!

Mark