Saw this on another board a member there is redoing a 1858.
You might want to try lapping the barrel out using an old method my gunsmith taught me. Take a 45-70 case and sharpen the mouth the best you can and cut some leather wads. You may want to use older brass because it will mess the case up. Then you take your roughly .457 diameter leather wads and use a leather punch to cut holes about a 1-4 inch in diamenter directly in the center of the wad. After you have about 6-8 wads you are ready for mounting. Take an 8x32 bolt that will fit into the threads of an older one piece steel cleaning rod (GI rods work great for this task) and place your wads onto the bolt. You have to measure the bolt to fit through your leather and have room to tighten down. Wads can be added or subtracted if needed. For a badly pitted barrel start with course grade lapping compound. Can be obtained at an Auto parts store in course and fine grade packages. Coat the leather wads with the lapping compound after snuging them into the cleaning rod. Starting on the breech end tap the coated leather into the bore. Run the lap that you just formed through the bore untill you see a little leather pass the muzzle. Tighten the 8x32 bolt one quarter turn or more. Too much will sieze, but a little will give you friction to do the job. Pull the lap back through the bore and stop when you see leather showing. Work the course grade compound and clean once in a while to see if the pits are gone. This will take a while, so clear your evening schedule. Once you get the desired results switch to the fine grade and take the same steps as you did with the course compound. If you are worried about damaging the bore don't. The lap will only take off very little, the worst that will happen is that you will not be able to get the pits out. In your case there is nothing to loose. You will know when the job is done when the bore looks like a mirror.
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