Let the bbl. soak overnight with Kroil and then hit it with JB paste. It won't hurt it at all and will probably remove the rust well. Kroil loosens rust pretty good as it's a good penetrant. JB, used per instructions, won't ever hurt a bbl. In fact, it will polish it some and prevent fouling where that rust was if it wasn't pitted. I always polish a new bbl. some with JB before shooting it to smooth it and help it break-in easier. It works well.