Originally Posted by
LRRPF52
That's what I said, and of course I know the difference between case hardening and through hardening. The extension that shattered the lip off appears to be through hardened in that thin area. Other extensions will only gall in that area at worst. This is the first time I have seen that happen.
What I've gathered from watching the Melonite/QPQ/Nitrocarburizing processes is that there is a compromise of sorts going on, and it all revolves around the barrel extension and gas port, aside from the metallurgy of the steels.
Since extensions are usually 8620 and case hardened, they will take deposition of nitrogen in a different way. The obvious question is, do I send barrels by themselves to be nitrided, or do I send them with the extensions attached?
If someone is sending them attached with the gas ports cut already, it's because they want nitriding in the port to reduce port erosion, but then there is the problem with surface geometry changing at the microscopic level enough to affect torque in the threads.
If a barrel maker doesn't know what they are doing, they might just be sending out pipes for nitriding with the already case-hardened extensions attached, but the process for the nitriding is geared towards the specific metallurgy of the barrel, which is going to be 4140 most of the time.
It leads us back to the original topic: How do I as a consumer know if the company is riding a trend without engineering know how, and how do I know if my extension or barrel is treated properly? One of the first things I noticed when Superior Barrels entered the market was that they specifically mentioned their barrels were not full auto rated.
Then I came across the paper from the 1950's where Army Ordnance tested barrels that had been nitrided, hard chrome lined, and even NiB treated, specifically looking at full auto endurance. Hard chrome lining had the best performance, resisting high heat and repeated shock forces better than the others.
This topic is important to me because I shoot a lot in extreme cold, where we push the material properties of steel in a state that is often more brittle. I'm not talking about going out and plinking on the weekend either in December. There are serious, regionally-significant aspects of this that are important to me and others. I know that MIL-B-11595E holds up well in extreme cold, high volume.