Rockwell hardness on finishes

[Singlestack Wonder]

My goto finish for many years has been Melonite QPQ (known under many other names, i.e. black nitride, etc.). It has a rockwell hardness of ~55 and is a surface treatment vs. coating, plating, etc. which adds to potential dimensional issues. Impervious to rust, heat, etc., it is now the goto finish for many firearms manufacturers. Back when I utilized 1911's, all were melonite qpq treated (if I touch a blued or stainless pistol, rusty fingerprints would appear in an hour).

http://www.blacknitride.com/frequent...questions.html

[lysander]

I look at it like this: the piston on every modern military small arm (since WWII, and even then many were) is chromed. The bolt on an AR is essentially the piston, so why not?

Why not?

Well, there are several reasons, depending on the exact type of chromium finish.

There are two distinct types of technical chromium electro-plating:

1) Hard Chrome plating (HC, AMS2406) - HC is NOT recommending for highly stressed parts like bolts. The reason is hard chrome plating, by the nature of its application leaves micro-cracks in the surface, which are stress risers. These are fatigue initiation points if the part is subjected to stress. Pistons are not subjected to very high stresses, bolt lugs are.

2) Thin Dense Chrome (TDC, AMS2438) - TDC is a different type of chromium plating that, well, is thinner and denser than HC. It also has a smooth crack-free surface, so its resistance to fatigue cracking is much better than HC.

TDC usually goes by various trade names such as Electrolizing, Armoloy, Electralloy, etc.

You can deposit a HC film as thin as 0.0002” however, that does not make it TDC, the difference between HC and TDC goes beyond what Specifications cover. It is related to chemistry, current density, temperature, etc. The deposit is different regarding physical and mechanical properties.

The original bolt and carrier for the AR-15 were never hard chrome plated. They were Electrolized (TM) by Electrolyzing Inc of Los Angeles, Not only was the process more expensive, but Electrolyzing Inc was unwilling to license their proprietary TDC process to anyone, and would have been unable to keep up with the high volume, full rate production of M16 would entail.

[Rogue556]

LMT is kinda cagey about what exactly the finish is. "Proprietary" is usually what you hear.

I noticed that, there were only a few places that even remotely hinted at what it actually was. I've been on the fence on ordering one for suppressed use while I wait for the stamp for my RC2. I may just have to do it.

.

Great comment, and thank you for the response. There are so many wonder coatings out there now that it's difficult to keep up with, especially when they seem so similar. It'd be nice if there was an in depth test done between the most popular choices, but I assume that would be a pricey endeavor.