That is an interesting problem! I gave it some thought and the best way in my opinion would be to heat them up in a controlled atmosphere oven. with a very thin ribbon of braze metal and flux in the joint. Held by a clamp of some sort. Yes, you would have to relay the ribs. When Parker(and others) did it, the barrels were in an unfinished state, so scale was not an issue. Try to do it with a torch would probably net you a crappy job. I might be willing to try a repair for myself if I could buy one of these guns cheap enough. Always wanted a steel shot special!
That was how barrels were brazed at Verney Carron when I was there. The barrels were assembled in a jig, everything was held in place with wire that was tightly wound around the bits, and the barrels run into the oven on a steel link conveyor. I saw the process, but, never saw the machine run. I got the impression they assembled barrels in fixtures Monday through Thursday, turned on the gas and lit the oven on Friday, and brazed barrels before the weekend.
Best,
Ted
