I once acquired a gun in that condition, the dent being on the outside if you looked head on at the muzzles. Having a strong bond with the muzzleloaders, an inventive DIY group if ever there was one, I bought a section of steel bar and filed it down to a nominal muzzle ID. Then I filed a slight flat along one side of the bar, just enough for the bar to slip into the muzzle past the dent. Gently rotating the bar back and forth ironed out the dent Gun looks fine and shoots to POA. I would not want to do it on a very big dent.
