I wound't say it is out of the question. A set of .450 barrels could conceivably be reamed out to .475 or .50 and re-rifled if there is enough meat in the barrels.
: I know of no substance that could be put in to fill the pitting evenly with the bore & then harden, unless Danzac will do that to sme extent. You can do a search with goggle for info on that stuff.
: I think perhpas you have the wrong idea about pitting. Mild pitting isn't too bad and can be polished to where it does no harm. The worse pitting is in the chambers, where the case can expand into them and not eject.
: The Mauser I did, was merely firelapped smooth due to the grinding compound imbedded on the bore-riding nose of the lead bullets I shot from it. It is a simple process. If both grooves and lands have pitting, then the entire bullet is imbedded in compound. There are kits available in the States for doing this with modern rifles and old ones alike.
: The coating I did on the Sharps, the Metaloy, becomes one with the surface of the metal with some penetration of that surface. Nothing can get under it, but the surface must be perfectly clean or it won't work.