This is what the guys at LWRC says.
Ferritic Nitrocarburizing Information
The standard M6 barrel
features a ferritic salt bath nitro carburized interior and exterior finish. This finishing process is also applied to the barrel
extension and gas port. This finishing process is a surface conversion process in place of industrial hard chrome and is therefore inherently free from nodules, flaking, pits, stripping, or anode burrs. The process results in chemical and structural composition changes that can be described as a case hardening to a depth of 0.005" of the alloy leading to increased surface hardness, lower coefficient of friction, enhanced surface lubricity, improved running wear performance, increased sliding wear resistance, enhanced corrosion resistance and heat resistance over industrial hard chrome. Since the process is a surface conversion and does not add material to the internal dimension of the bore, the barrels can be hammer forged on size over a very precise polished rifle mandrel. With hard chrome, the bore must be manufactured oversized or honed and electro-polished to make the interior dimension oversized making room for the chrome plating. This plating is inconsistent which is the reason sniper or target barrels are typically not chromed. Nitrocarburizing has proven superior as a surface finish to phosphating to oxidization when subjected to MIL-STD-810f salt fog/spray testing and is a scratch resistant flat black finish.
In addition the barrel
extension and gas port are also surface converted minimizing barrel
extension wear, increasing corrosion resistance and limiting gas port erosion. Cross sectional analysis done by HP White Laboratories before and after nitro carburizing proved the surface conversion process does not negatively interfere with the heat treat or temper of the barrel
extension. LWRCI contracted HP White to conduct these tests.
The ferritic salt bath nitro carburizing process has been proven by independent testing to be superior to chromium plating both it terms of corrosion resistance and abrasion resistance, lubricity and wear. It has been long adopted by FAMAE of Chile for use in all government small arms, and also by the Polish Military. It has been adopted by Sig Arms for use in their carbines, Glock in their pistol bores and various other large military arms manufacturers with excellent results.
And this is from a trade publication
FERRITIC NITROCARBURIZING accomplishes surface treatment of
a part in the ferrite region of the iron-carbon equilibrium diagram (Fig. 1).
As the process takes place in the ferrite region, both nitrogen and carbon
diffuse into the steel surface. The process is categorized as a thermochemical
treatment and is carried out at temperatures between 525 and 650 °C
(975 and 1200 °F); the typical process temperature is approximately 565 °C
(1050 °F). The purpose of the process is to diffuse nitrogen and carbon
atoms into a solid solution of iron, thus entrapping the diffused atoms in the
interstitial lattice spaces in the steel structure (Ref 1).
As with the nitriding procedure, there are many methods and derivatives
of ferritic nitrocarburizing. These are discussed in the chapters that
Ferritic nitrocarburizing improves the surface characteristics of plain
carbon steels, low-alloy steels, cast irons, and sintered ferrous alloys. As
described in later sections of this chapter, resistance to wear, fatigue, and
corrosion are improved with the introduction of nitrogen and carbon.
means the resistance to wear on the metal surface.
This is accomplished by changing the nature of the surface compound
layer, which is also known as the white layer. The completed
compound layer will form with both epsilon (ε) and gamma prime (γ ′)
phases. The dominant ε-phase resists abrasive wear.
of steel are greatly improved by altering the composition
of the compound layer. This means that treated steel has greater
resistance to fatigue failure than an untreated steel (Ref 1).
After ferritic nitrocarburizing, steel parts can
withstand many hours in a salt spray environment, whereas an untreated
plain carbon steel will fail the corrosion test very rapidly.