HK and others have made reference to worn out gun barrels. Why not
recondition them by plating the inside with electroless Nickel
followed by the electroless nickel Boron nitride alloy? This might
work best for shotguns without rifling but even rifled barrells might
work this way.
How deep is the rifling on a barrel?
How does a barrel wear out, does it actually get thinner or simply
develop cracks. I dunno a way to repair cracks.
You'd think someone may have tried this but I find no references to it
on google.