When I was given a rifle that suffered similar circumstances from a friend. I took some water soluble valve grinding paste that I buy at NAPA Auto Parts. I took two new bore brushes and one old one. After I attached the old one, I then wrapped a patch around the bristles. To the patch I applied a good coating of valve grinding compound. I then basically lapped the barrel. I gave it 25 strokes. I then changed patches and gave it another 25 strokes (a stroke being all the way down and then back up). I ended up giving the rifle 100 stroked then gave it a good soap and water bath. After that I scrubbed the barrel with a new brush and some Birchwood Casey Bore Scrubber. After that I ran solvent patches through the rifle. The best I could get out of them was a dull gray color.

I then shot the rifle and discovered that although the bore was pitted, and the outside was rusted (which I later cleaned also) the rifle shot excellent groups. After shooting I cleaned the rifle again with a water bath and more solvent. Then since you took the barrel back to bare metal it is very important that you apply a high quality gun oil on and inthe barrel good and heavy. I used Rem Oil with Teflon.

After the rifle had a good coating of oil I took a clean cotton cloth, placed it on the floor and set the rifle muzzle end down onto the cloth so the heavy coating of oil would drain away from the breech across the damaged bore and finally come out the muzzle onto the cloth. I was surprise in the next morning to find no rust color in the oil, so I suspect the lapping, shooting, and constant cleanings at least stopped the rust from doing more damage.

If your worried about a hard lapp or the pitting and rust do not look bad. You can clean the bore in a more gentle manner with Scotch Brite Scouring Pad and some J-B Bore Paste. Just cut a small amount of the pad, attach it to a patch worm, and apply bore paste. Then scrub the barrel that way...

Hope all comes out well for you and your rifle.