Barrels are subjected to extreme stress and no barrel (that's used regularly) will last forever. To help increase the service life of rifle barrels, some are chrome lined. This is done to reduce wear (especially for rifles fired at a very high rate on a regular basis -- repeated large ...
It's probably still not something I would care to play around with. Lowers are cheap enough now that I think I'd just buy myself a pistol marked lower if I wanted to do my own AR pistol build. It doesn't have to be marked in any way. You will just be paying for a stencil. ...