I once read of a guy who had his German drilling run over by a milk wagon (back when they were horse-drawn and had steel rimmed wheels) Both the Boehler Blitz shot barrels were flattened, but a gunsmith managed to repair them to a state where you couldn't tell they had been damaged.
I once did a, mostly cosmetic, repair of a Simpson Suhl which had developed a bulge just behind the choke. I used the metode Gebirgsjäger describes, using an internal plug and an ordinary peen hammer (flat side, of course) The repair was still visible, as the barrel had some spring-back, and the part of the barrel under the ribs still had a bulge, but the owner was happy with the result.