I had a customer that had a cap lock gun and the mechanics were good, but the barrels were rusted half away. It was a challenge but two old discarded barrels from guns that had the mechanics shot were dismantled, breech plugs from the original gun were installed and the barrels were cleaned and the rib soldered on. It was impressive build. Then I got a hold of a copy of the Recreation of the shot gun and found the idea wasn't new at all.
After getting this book I had to have The recreation of the Kentucky Rifle. Great books, highly recommend.
I buy all the old cartridge relics at auctions and from individuals. 10 to 50 bucks for good barrels I feel is on the cheap, plus a pile of good salvageable parts.