ramshackle wrote:My best guess is a borate added to the molten metal.
Okay, over to the mob.
Molten metal? You mean during smelting the iron into steel ingots for forming into bars that then become barrel blanks? That would mean you could machine the surface with a lathe and still maintain the finish? It would also mean the finish would be the same inside the bore?
I think it's much more likely a hot oxide bluing technique that _might_ add boron to the surface during the rusting process.