20 bore lifter

[David Noble]

That is a great gun you have there. Everything checks out to the book.
Have the barrels measured by a professional gunsmith, one who is knowledgeable in shotgun barrels. If the barrels are thick enough in the eight to ten inches ahead of the chambers it's possible that area could be honed enough to remove a great deal of the pitting and still have enough strength for shooting reduced pressure loads. I personally feel that would be of more benefit than sleeving.