XM16E1 barrel erosion study (1966)

[MegademiC]

Lysander (or whoever knows metallurgy), do NP3 and other popular coatings/surface treatments also have this downfall?

No. Hydrogen embrittlement is one issue, but easily reduced with post-processing, if done properly.
Hard chrome has micro-cracking which leads to propagation. Nickel (electro and electroless) does not have micro-cracking.

Hydrogen embrittlement is less of an issue with Ni than Cr, and even less with elecroless Ni.

[sinister]

Then, nitriding avoids all these issues?

Check out this older thread:

http://https://www.m4carbine.net/showthread.php?171789-Melonite-vs-Chrome-Lined-Longevity&p=2154035&highlight=#post2154035

[ABNAK]

No. Hydrogen embrittlement is one issue, but easily reduced with post-processing, if done properly.
Hard chrome has micro-cracking which leads to propagation. Nickel (electro and electroless) does not have micro-cracking.

Hydrogen embrittlement is less of an issue with Ni than Cr, and even less with elecroless Ni.

In 2019 anyone worth their salt hard chroming gun parts would be addressing this issue in their processes. Chris Peters of Metaloy and I had a phone discussion a few years back on this subject (I have just about all my AR bolts hard chromed by him). When I mentioned hydrogen embrittlement he seemed almost irritated, but quickly explained why it used to be an issue and, more importantly, why it shouldn't be an issue today. While I don't recall the exact details of the conversation I do remember him mentioning a "window" of time post-chroming for the stress relief (?) to take place and therefore eliminating HE.

[ABNAK]

Doesn’t LMT use hard chrome on their enhanced bolt? I also seem to remember the Mk46 bolt being plated with some shiny stuff; don’t know what.

LMT won't disclose what the coating is on their Enhanced bolts. The newer ones indeed look like hard chrome (duller finish) but the previous generations were shinier; rumor had it was some kind of an electroless nickel?

[Slater]

Reading the test report, some additional notes:

"The test bolt carriers made of through-hardened steel failed after about 10,000 rounds each as compared to no failures in the standard carriers. The failures were attributed to increased wear in the area where the bolt sealing rings slide in the carriers due to the reduced hardness of the test carriers as compared with the case-hardened standard carriers.

The test magazines, both 20 and 30-round magazines, functioned well and reliably; however, those magazines that did not have protective finish showed severe pitting after exposure to a brief shower, and severe corrosion of the spring.

The screws fastening the carrier key to the bolt carrier loosened after firing approximately 10,000 rounds and had to be checked, tightened and restaked where necessary every 1,000 rounds. Failure to do so would result in malfunctions and breakage of the screw heads.

The charging handle would sometimes fly to the rear in automatic fire and strike the shooter in the nose. Although this would not injure the shooter, it would startle him, and this condition should be eliminated. This malfunction would usually occur when there was a broken or jammed ring spring in the spring guide.

The weapon receivers got very dirty from firing due to the gases being vented from the bolt and carrier, and frequent cleaning was required to prevent malfunctions. The use of oil in the gas chamber only adds to the carbon formation and in- creased malfunctions and should be avoided.

There was only one failure of the standard firing pin retaining pin in the test and no failures of the test retaining pin."



I haven't heard of any instances nowadays of the charging handle flying back and impacting the shooter. Presumably that's a non-issue these days?

[ABNAK]

Then, nitriding avoids all these issues?

Show me one military on this planet that uses a nitride barrel on their issue weapon. Nitride is a cheapskate shortcut that is popular with manufacturers now because it's, well, a cheaper way of doing it. I would take a nitride barrel over a non-treated one but NEVER over a properly hard chromed one. YMMV.

[lysander]

Show me one military on this planet that uses a nitride barrel on their issue weapon. Nitride is a cheapskate shortcut that is popular with manufacturers now because it's, well, a cheaper way of doing it. I would take a nitride barrel over a non-treated one but NEVER over a properly hard chromed one. YMMV.

Well, this thread was originally about bolts and bolt carriers, barrels (in general) are not fatigue sensitive, so the down side of chrome plating is non-existant.

But, to answer your question, the US. For 19 years they used a nitrided barrel in the M242 25mm Bushmaster cannon.



The Enhanced 25mm recognizable by the heavier fluted barrel and the more cylindrical muzzle brake, introduced chrome plating to the bore. This was first fielded in 2000 in the M2A3.

[mark5pt56]

I could be wrong, but one thing that stands out is "oil in the gas chamber adds to carbon accumulation" Reference then was carbon based lubes and I still hear this today that "adding oil just makes more carbon" Well, we have synthetic lubes today, not an issue in my experiences.

I know the thread intent has morphed, but good information flow.

[ABNAK]

Well, this thread was originally about bolts and bolt carriers, barrels (in general) are not fatigue sensitive, so the down side of chrome plating is non-existant.

But, to answer your question, the US. For 19 years they used a nitrided barrel in the M242 25mm Bushmaster cannon.

The Enhanced 25mm recognizable by the heavier fluted barrel and the more cylindrical muzzle brake, introduced chrome plating to the bore. This was first fielded in 2000 in the M2A3.

Well, I was specifically referring to issue small arms, but even the 25mm has been "Enhanced" with chrome lining. Wonder why?

Now Stellite? A whole other issue that I would say is superior to chrome lining. Would be cool if someone made an AR barrel lined with Stellite. It'd be pricey as hell but I'd buy at least one.

[lysander]

Well, I was specifically referring to issue small arms, but even the 25mm has been "Enhanced" with chrome lining. Wonder why?

Now Stellite? A whole other issue that I would say is superior to chrome lining. Would be cool if someone made an AR barrel lined with Stellite. It'd be pricey as hell but I'd buy at least one.

Stellite lining actually has a lot of drawbacks.

1) Instead of one hunk of steel with a precision round hole that is perfectly straight, you need at least three parts, usually four, parts with not only straight precision holes in them, the outside has to be held to excruciating tight tolerances.

2) The liner is normally only a third the length of the rifled bore, so there is a discontinuity in the rifling. As much as 0.010".

3) Because the chamber, throat and barrel are three different parts, good luck keeping all the bores perfectly concentric. You can see that in the drawing that they weren't expecting better than 0.010" T.I.R. chamber to throat and 0.001" T.I.R. throat to bore.

There is a reason the military has drifted away from Stellite lining, in favor of the much simpler and cheaper chromium plating. The cost of making a built-up, stellite lined barrel, far exceeds the cost of the two or two-and-a-half chrome-plated barrels for the same life.