I have seen old wincester lever guns where they "saved" the barrels by reaming them as far out as they would go and then lined them with new chamberings.

Springfield 1861 muzzle loaders were converted to trapdoor like this using the Allen method. Barrels and liners were simply brazed at the chamber and at the muzzle.

I like the idea of switching to a more common caliber instead. Say, 243 or maybe something like a 30-30 Winchester.