Nitride Finish
Also: Melonite · QPQ · Salt Bath Nitride
A surface-hardening treatment that diffuses nitrogen into steel for wear and corrosion resistance, applied to both the bore and the exterior.
A nitride finish, sold under names such as Melonite and applied by a salt-bath QPQ process, diffuses nitrogen into the surface of the steel rather than plating a coating on top of it. That builds a hard, slick skin a few thousandths deep that resists wear and corrosion far better than bare metal. Because it treats the barrel inside and out, both the bore and the exterior gain protection in one operation.
Unlike a plated lining, nitriding does not add a separate layer of material to the bore, so it changes interior dimensions very little. The harder, smoother surface gives residue less to grip, which can slow fouling and ease cleaning. It serves as both a rust barrier, like the older bluing it often replaces, and a durability upgrade comparable in spirit to a chrome-lined bore.