There is a tool called a hydraulic dent remover that is used to remove the dents in tubing. There is a variation that is used to remove dents in classic side-by-side shotgun barrels that are dented in falls. They work slowly and can raise dents in hardened steel as well as in old layered damascus steel and be compatible with the interior tapering of the tube for the choke. These classic shotguns barrels were silver-soldered together with a rib and the tool is designed to work very slowly and to not deform the tube and possibly compromise the silver solder, as well as not damage the tapering of the choke. The tool can be had with different diameters. This would not be something most gunsmiths of modern arms would have. There may be something similar that a framebuilder would know about.
A long shot, but another possibility.