It had to have been Titanium Nitride coating.  The same nice and pimpy gold color coating that is available on Desert Eagles and that Hornady uses on the sizing ring of some of their reloading dies.  There is at least one maker of AR15 bolts and carriers that also offers them with a TiN finish.

Hardness > 2000 kg/mm2 Knoop or Vickers Microhardness.  Values of 2500-3000 are typical.  Equivalent to approximately 85 Rc.  Harder than hard chrome or carbide material.

The coating forms a metallurgical bond to the substrate that will not flake, blister, chip or peel.  In fact, the coating is actually implanted slightly into the surface layer of the substrate.  Several adhesion test methods are possible, contact BryCoat for more info.

TiN generally provides low friction against steels, carbides, TiN, ceramics, platings, etc

Highly inert to acids, bases, solvents, caustic, etc.

Benefits of Titanium Nitride Coatings

Wear resistance
Maintaining sharp edges or corners
Prevent galling, seizing or cold-welding
Provides an attractive metallic gold color
Safe for surgical devices and food processing
Decreases friction
Resists corrosion
Withstands high temperatures

Features of TiN Coatings

Extreme hardness.  Harder than carbide.
Lubricity
Resistant to nearly all chemicals
Heat resistant
Non-stick surface
Non-toxic
Environmentally friendly process