Something else to try. Buy a pint or so of naval jelly and fill the bore. Let it stand for an hour or so and then do the other barrel. Stuff will get down to the very bottom of the pits and remove rust. Careful though, it will also remove bluing. If you then oil the bores after shooting those pits will never rust again. I've used the stuff on steel in a marine application and have been amazed at the results.