Nitrided barrels are all the rage these days. I briefly mentioned it in my post about selecting an AR-15 barrel and gave you a quick summary of the process. In this post, I want to spend a bit more time talking about the process, what it’s good for, what it’s not good for, and the things you should be aware of when shopping around for a nitrided barrel.
Here is a quick description of the process from an industrial coating company, IBC Coatings, I’ve bolded some key elements.
Salt Bath Nitriding/Nitrocarburizing was originally created as an alternative to gas nitriding that would produce a more uniform case through surface contact between the substrate and liquid salt. When steel parts are placed into a preheated liquid salt, there is sufficient energy localized near the surface due to differences in chemical potential that then allows nitrogen and carbon species to diffuse from the salt into the steel substrate. The process is carried out at 750-1050°F, making it faster than gas nitriding. Lower temperature cycles produce an S-Phase/Expanded Austenite case in stainless steels. Post-oxidation after nitriding combined with polishing produces coatings with exceptional appearance (black color) and high corrosion resistance (greater than electrolytic chrome plating). To ensure part quality, our salt baths are continuously monitored, with chemistry adjustments made when necessary.
Salt Bath Nitriding/Nitrocarburizing is well known under various trade names, including ARCOR®, TENIFER®, TUFFTRIDE®, MELONITE®, and QPQ®.
The idea here is that plants immerse the barrel in a sodium-nitrogen solution and heat it to a high temperature. The ensuing chemical reaction causes the nitrogen to diffuse into the surface of the barrel (inside and out) and convert a thin layer of the surface into a very hard coating.
The surface of a nitrided barrel is in the realm of 60 to 65 Rockwell, while the typically gun barrel steel is 28-32 Rockwell. This surface layer becomes a very corrosion resistant “case” around the barrel steel.
This video shows what the process looks like at the Melonite plant.
The process of nitriding is fundamentally different from chrome lining. With chrome, a thin layer of the actual material is removed and then replaced with hard chrome. Nitriding actually converts the surface into something else.
Benefits of a Nitrided Barrel
The primary benefit of the nitriding process, from a rifle standpoint, is corrosion resistance. Barrels that undergo the process are extremely resilient in the face of corrosive environments, nearly four times more so than chrome.
Along with that is that the entire barrel, both inside and outside, undergo the same process. In effect, the entire barrel exterior becomes resistant. Chrome, on the other hand, only protects the bore of the barrel.
Additionally, the surface layer created by the nitriding process has a much lower coefficient of friction compared to bare metal or chrome. It’s not quite as relevant to barrel bores, but this means that you would need less lubrication on treated parts.
You might think that reducing friction might increase velocity as well. I have read some accounts verifying this on Accurate Shooter, but it was only by about 1% or so. I don’t think increased velocity is a selling point here.
Cost is a third benefit. Nitriding is both easier and cheaper to perform than chrome lining.
The bottom line is that the process costs less to perform, which means cheaper barrels. Now, that’s not to say that you’re automatically going to pay less. You might, depending on the manufacturer, but there are still a lot of QC procedures that go into this. I’ll talk about that in a moment.
Barrel Nitriding vs Chrome Lining
This is where the rubber meets
The real benefit of nitriding is that you get better corrosion resistance than chrome, on both the inside and outside, at a lower cost, without affecting accuracy.
Remember, the surface of the bore alters its chemical structure to become a harder material. That is different from chrome, where we are adding a new layer of something. Since it’s a conversion process rather than an additive process, nitriding maintains the exact dimensions of the bore before and after.
Consistency is accuracy.
I reached out to the Stephanie Dahlke, the Operations Manager at Criterion Barrels with a few technical questions. She brought some of the engineers to comment. They mentioned that it’s possible to maintain very accurate barrels with the nitriding process because they can machine the barrel, polish and lap the bore, then send it off to get nitrided. The barrel comes back with the same polished finish it left with but much more protected.
If you’re following so far, then it seems like nitriding is a fantastic deal. You get all of the benefits of chrome, and then some, while keeping accuracy. All the while it costs less.
Unfortunately, there are no free lunches. There are tradeoffs and compromises.
Nitriding is about a century old and well known in the weapons world. So why haven’t we seen it used in more weapons? The first reason is heat resistance. The surface of a nitrided barrel may be harder than chrome, but it is not as heat resistant.
In 2002, Boeing performed a series of tests comparing barrel finishes during heavy firing schedules. That particular test used variants of the 25mm Bushmaster automatic cannon. Results clearly showed that chrome linings won out as heat built up
Another US Army study performed in 1967 gives a little more context. This second study compared barrel life of different linings through a 5.56 machine gun. The barrels averaged a firing schedule of 200 shots per minute. That’s almost the equivalent of firing seven 30-round magazines per minute.
The nitrided barrels lasted an average of 28,000 shots before rejection. In this study, rejection happened when 20% of the shots in a given string of fire showed yaw of more than 15%. Yawing happens as a barrel’s rifling fails and can no longer stabilize the bullet.
Put another way, that’s the equivalent of firing 933 30-round magazines over the course of about two hours.
Now, keep in mind that we’re talking about some extreme conditions with a very generous standard for barrel rejection. You would likely get rid of a barrel for decreased accuracy long before it got to the point of bullet tumbling.
But anyway, all of that to say that the average rifle shooter probably doesn’t need to worry so much about heat generation in a semi-auto rifle.
By the way, there were other barrels in that test. The ones that were bare metal, without any linings, lasted just under an average of 9,000 shots before rejection. The top performers in the test were an experimental configuration of a chrome bore with stellite insert by the chamber. Those went for almost 39,000 shots before rejection.
Nitriding Process Risks
Referencing the quoted portion above, the nitriding process takes place at a temperature of 750 to 1050 degrees Fahrenheit.
Coincidentally, this is near the same temperature used for barrel heat treatment and stress relief, which ranges from 950 to 1200 degrees depending on the type of steel.
There’s a very real risk that heating to such temperatures can undo heat treatments already performed by the manufacturer.
I asked Stephanie about this. She relayed that it’s very important for the nitriding facility to certify the temperature they used. The particular facility Criterion uses is ISO certified and provides all of that documentation for them.
While I haven’t seen anyone mentioning decreases in accuracy after nitriding, I have seen many warnings to not perform the process on a barrel that’s already been fired a significant amount. The microcracks formed in the bore and chamber of such barrels may be aggravated by the high temperatures. That could actually degrade the barrels accuracy.
Barrel Extension Torque
Another thing to consider when shopping around for nitrided barrels is the barrel extension itself. If the barrel and extension are nitrided together, the process tends to relieve the torque between them. Manufacturers need to take care to re-torque and headspace the extension and barrel shoulder after nitriding is complete.
The very hard surface of a nitrided barrel means that it’s hell on drill bits. When I was finishing up one of my rifles that happens to have a nitrided Faxon Gunner barrel, I wanted to have the front sight post pinned in place. It took me months to find a shop willing to do the work.
Finding shops ready to drill and pin a standard phosphated barrel is easy, though. If you’re wondering, I utilized W.A.R. Rifles in Manassas Park, VA for the job. They did very clean work and I would definitely recommend them in the future.
Nitriding Stainless vs 4140/4150/CMV Barrels
Now you have a sense of what nitriding is and what it’s good for. The last thing I wanted to talk about was the market. When you go out and shop for a nitrided barrel, you’re going to see both standard 4150/CMV and stainless options.
For a long time, I didn’t understand why a company would go through the effort of nitriding a stainless barrel. If the primary benefit was corrosion prevention and nitriding was a replacement for chrome lining, I didn’t think it made sense to perform the process on a barrel that you weren’t going to to use chrome on anyway.
So, I asked Stephanie at Criterion about that one as well.
While corrosion resistance is still a benefit, even for stainless barrels, the primary advantage is the hardness of the surface layer. Again, since nitriding is a surface conversion process, a barrel manufacturer can go through all of their regular processes to make an accurate and polished stainless barrel, and then help it last longer with nitriding.
That said, you’re going to find cost savings with well-made nitrided chrome-moly barrels. Either one will serve you well from an accuracy standpoint. However, a nitrided chrome-moly barrel is still your best bet over stainless if you want to go with a light profile. Just because it’s nitrided doesn’t mean it overcomes the temper embrittlement issues inherent in stainless barrels.
Well there you have it. That about wraps up everything I know about the nitride process as it applies to rifle barrels.
The two primary benefits of the nitriding process is corrosion resistance and maintaining a barrels dimensional consistency, which improves accuracy. The surface is also very hard and protective, provided you don’t abuse the barrel with excessive heat.
If you plan on doing excessive belt-fed levels of fully automatic fire, then a chrome-lined barrel is still the way to go. For most of us, a nitrided barrel will work just fine. By bottom line, as always, comes back to selecting your barrel from a quality manufacturer and letting them worry about how it gets made.