Parkerizing
Also: Phosphate Finish
A phosphate conversion coating that leaves steel a matte gray-black, corrosion-resistant, and porous enough to hold protective oil.
Parkerizing immerses steel in a hot acidic phosphate solution that chemically converts the surface into a layer of crystalline phosphate, leaving a matte gray to black finish. The coating is porous, which is the point: it soaks up and retains oil, and that trapped oil does much of the corrosion protection. The military leaned on parkerizing for decades because it is rugged, cheap, and forgiving in the field, and it remains common on utilitarian rifles and parts.
Like bluing, parkerizing converts the surface of the steel rather than laying a film on top, but its rougher, thicker texture holds oil far better and resists wear more aggressively. It predates modern options such as a hard nitride finish or a sprayed cerakote coating, yet it is still a sound, no-nonsense protective treatment for a barrel or action.