You could make your own chamber inserts or sleeves. You shouldn't worry about anything ahead of the chamber if the gun is in pitted condition. It isn't a collector condition gun. I would solder or glue in a sleeve of proper size, and shoot 12 gauge shells. You don't need to modify the extractor unless it is badly oversized. If this is the case, use an eccentric sleeve that pushes the shell toward the extractor. The alternative is to spend a bunch of money to ream out the pits entirely, then spend a bunch more money to purchase Briley fitted tubes to 20 gauge with screw in chokes. Do you like the gun that much?
