The current drawings call for hard chromium plating inside the key, and inside the carrier. These are not highly stressed areas, and would benefit from the retention of oil in microcracks in the plated surface.
Nobody in their right mind would hard chromium plate a bolt. These guys know enough about their business to use some form of TDC.
I have several bolts (mil-spec, check-the-boxes ones) that I have had hard chrome plated by Metaloy. I talked to Chris once for probably 10 minutes about the process. I was primarily concerned with hydrogen embrittlement at the time. I also have a factory Daniel Defense BCG.
Now, I have very few rounds through any of them compared to some of you folks, but have owned chrome bolts or groups for many years and have never had any issues thus far. Did I put 5K rounds through any one of them? Nope. YMMV of course…and they certainly are easier to clean! 
Back in the early 90’s I bought a Smith Enterprises chromed BCG. That unit was swapped to a couple of different guns when I sold the host weapon but wanted to keep the BCG. Maybe a couple thousand rounds overall. No problemo. Once cleaned it looked brand new as you can easily see any black carbon with a satin chrome finish. Mechanically I never had any regrets.
I hold your tech opinions in esteem, so not trying to bicker. Just relaying my personal experiences.
Chromium plating IAW SAE AMS 2438 - “Plating, Chromium (Thin, Hard, Dense)” or SAE AMS 2460 - “Plating, Chromium”?
It is well known that chromium electroplating is related to the reduction in the fatigue strength of base metal . . . . something that is not an issue with Thin, Dense Chromium Plating.
Effect of Chromium Plating on the Endurance Limit of Steels Used in Aircraft
Effects of surface treatments on the fatigue strength of AISI 4340 aeronautical steel
On the Effect of Hard Chromium Plating on the Fatigue Limit of Steel
Study the Effect of Chrome Coating Thickness to Fatigue Strength of the Axle-Shaped Machine Parts
Fatigue Strength of Chromium-Plated Steel
The effect of chromium plating on the endurance limit of 4340 steel
What companies offer the THD coating? PM if you don’t want to seem like you’re shilling.
I don’t know, honestly, I have never felt the need for a chrome plated bolt.
But if I did, I would write and ask the company which SAE specification they use to plate, and the plating thickness. I would hope all of them that offer chromed bolts would use SAE AMS 2438 and a thickness between 0.0002 to 0.0005".
If you want a company to plate something with TDC, you can google “thin dense chrome” and get an impressive list of people that do that sort of thing.
The microcracks seem to be an issue with NiB as well.
Oil will migrate as the parts warms up.
Any evidence of that?
I talked to John Noveske about BCGs one night while we were drinking beer and watching a river. When asked what type of BCG he thought was best, he replied phosphate coated “just like God intended”.
He was of the same opinion and for the same reasons.
Are we talking Hydrogen embrittlement with NIB?
[Fatigue Performance of Electroless Nickel Coatings on Stainless Steel Gas Turbine Compressor Rotors](file:///C:/Users/KC/AppData/Local/Temp/22097997-MIT.pdf)
Chapter 4 - The Properties of Electroless Nickel
Effect of Electroless Nickel Plating on Fatigue Strength of 30CrMoA Steel
Hydrogen embrittlement is not the only factor that causes loss of fatigue life in plated parts.
Yes. MarkM had one fail that he sent to me.
We analyzed it (cross sectioned) in our mechanical lab and found multiple small cracks beginning to propagate from the coating down to the substrate.
It was after the failure, so I don’t know if the cracks were initially there or formed due to the brittleness of the coating, but there were some even on the flat surfaces.
I am also of the opinion the morphology of the coating is a factor. It has a long, vertical grain structure, and is abbrasive.
Got it. So you saw a sample of one.
As far as the grain structure is concerned - That is one of the reasons NiB has a lower coefficient of friction (not a greater one and certainly not ‘abrasive’). When there is less metal touching between two surfaces you get - Less friction.
The early production of the M16A1 Rifle had an electrolized chrome surface treatment on the bolt-carrier group. Subsequently, it was found that a surface treatment of manganese phosphate was more durable (and cheaper), and it was adopted. In 1968, at the request of Project Manager-Rifles, a test was directed to evaluate a new process electrolized bolt-carrier group treatment with respect to reliability, durability, maintainability and safety of operation.
Final Report on Product Improvement Test of Electrolized Bolt Carrier Group for the Rifle, 5.56mm, M16A1
Abstract
A product improvement test of the electrolized bolt-carrier group for the rifle, 5.56-mm, M16A1 was conducted by the Materiel Test Directorate from 10 May 1968 to 10 July 1969. Additionally, in conjunction with the above testing, firings were conducted at temperatures of 95° F and 155° F using ball propellant-loaded ball and ball and IMR propellant loaded tracer cartridges fired as a mixture. Testing covered in this report was conducted at Aberdeen Proving Ground, Maryland. The purposes of this test were to determine the advantages, if any, of the electrolized bolt-carrier group over the phosphate-coated bolt carrier group, and to evaluate the compatibility of the M16A1 rifle with various cartridge mixes of ball and IMR propellant loaded tracer cartridges with ball propellant loaded ball cartridges. All testing was conducted in accordance with the methods paragraphs in section 2 of this report and with MTP 3-2-513 (Interim Pamphlet 20-20). One deficiency and two shortcomings were found pertaining to the ammunition. There were none attributable to the weapons. The deficiency was projectile break-ups when firing tracer cartridge lot No. LC12114, after conditioning at temperatures of 95°F or 155° F. The shortcomings were excessive projectile yaw when firing tracer cartridge lot No. LC12114, after conditioning at temperatures of 95° F or 155° F and ball cartridge lots Nos. RA5318 and RA5356 after conditioning at 95° F or 155° F (all three lots were loaded with ball propellant, WC846). It was concluded that the electrolized bolt carrier group was at least equal to the phosphate coated bolt carrier group. It was recommended that the test electrolized bolt carrier group not replace the present standard group unless there was a significant reduction in cost or the user had a specific requirement for such a group.
Fascinating info.
I’ll stick with the phosphate.
Thank you!
SIONICS Np3 BCG is my lone exception.
How does Melonite or the dozen other names company’s use for Melonite hold up?
I have 2 AR’s with Chrome linined barrels, they are the 2 Colts that were my Dad’s, all my other AR’s have Melonite, or a type of Melonite coating on the bore. Some of my B.C.G’s have Chrome lining and some don’t, I also don’t have any Chromed B.C.G’s there either Melonite or Phosphate coated.
I have never been in the Military & was never a LEO I am just a dude that loves to shoot cool Firearms, it doesn’t matter if the firearm was $125 or $12,000 I like what is cool to me. I do take my family’s and myself safety seriously so I do quite a bit of training and I love to take S.D classes. I reckon I have been lucky or whatever but the only problems I have had in classes were either ammo & or mag related, or not enough lube. Like I said I am no ways a expert, & why I try take as many classes as possible, I have dang sure not seen it all so I am asking because I just want to learn.
Thank you…
Don’t know, nobody has bothered to test it.
Well that sucks.
Thank you anyway!
You’ve got to remember that there is a difference between treating the metal like QPQ does, and coating the metal like NP3 or chrome. I understand this is a simplification, but the point still stands, they are two different things.
I would also like to point out that different treatments can work very well for different applications. Chrome and various other carrier treatments aren’t known to have issues, while those same treatments can create issues on the bolt.