Home Forums Hunting & Shooting AR and Tactical Rifles an education in chromed barrels!

I've posted this before, but not on the AR forum I believe. Carlos probably has more knowledge than all of us together.His email is below.


Butch

I was the Armorer for the Army Reserve Shooting Team for over a decade so I do have quite a bit of experience with both processes.

As I am sure you know, most G.I. barrels are made from chrome molly steel which is more susceptible to corrosion than stainless steel. Chrome lining is used on G.I. bores both to extend their shooting life and to protect them from corrosion that can be a problem in battlefield conditions where maintenance is sometimes sporadic or insufficient. Chrome lining does a pretty good job of protecting battlefield weapons. One of the objections to chrome lining is that it is thought to decrease accuracy. This seems to be a valid criticism and is backed up by machine rest tests I have conducted of identical barrels (same manufacturer but half chrome lined and half not).

As you are aware most barrel "wear" is in the throat area. So eventually the hot gasses from the burning of the gunpowder will eat thru the chrome lining at the throat. It is rumored that at this point accuracy will plummet but I have not found that to be true. (Or if true, it is overstated or maybe only occurs for that short period when there is both chrome and bare steel in the throat simultaneously - just at the point of initial break thru.) Chrome lined barrels can continue to shoot well for thousands of rounds after the bare barrel steel at the back of the barrel (throat) has been exposed due to erosion of the chrome lining. Another criticism of chrome lining is that it can flake off later in the life of the barrel resulting in poor accuracy. Obviously, this could also cause maintenance problems if the user is depending on the chrome to ward off corrosion and thus is careless in his bore cleaning. If corrosion is allowed to occur pitting will result and that will ruin accuracy for sure.

Barrel pitting was one of the reasons I got involved in Salt Bath Nitriding. I was loosing nearly as many expensive match grade barrels to improper maintenance (causing pitting) as I was to wear out. This was under the relatively benign target shooting conditions. Obviously given the reputation of degraded accuracy, using chrome lining wasn't an option. So for the past couple of years I have been Salt Bath Nitriding all of my match barrels and haven't had a single one exhibit any pitting. During that 2 yr. period shooters have put anywhere from a few hundred rounds to thousands of rounds on said barrels. I don't know how long the coating will persist so at this point I am still evaluating it as a preservative. I don't know what will happen in another year or two when these barrels get more wear on them. Salt Bath Nitriding goes on both inside of the bore and on the outside surface. So, instead of 2 manufacturing steps you have combined them into one. Salt Bath Nitriding doesn't degrade accuracy one iota, unlike chrome lining. This was the first thing that I verified when I began using the process. I broke in a bunch of barrels and then machine rest tested them for group. I recorded and kept the targets, cleaned up the barrels, and sent them to MMi TruTec for the Isonite process. When they came back I reassembled them on the same receivers with the same torque settings, same bolt carrier assemblies, same flash suppressors, etc. Then they were retested with the same ammo lots. NO degradation in accuracy and about a 1% increase in muzzle velocity.

Chrome lined barrels seem to clean up rather easily after a range session. I found the ease of cleaning of Isonite coated barrels to be similar to chrome lined barrels. The Isonite barrels clean up the easiest of any non chrome lined barrels I have ever used. Isonite can be applied to either stainless or chrome molly but the factory needs to know what steel you are sending them because the application process varies a bit. Again, I only have about 2 yr. of cleaning experience with Salt Bath Nitrided barrels. I don't know if the ease of cleaning will continue as the barrels get more rounds on them. Generally speaking, most non coated barrels get harder to clean later in life. Although stainless has a reputation of being corrosion resistant it isn't corrosion proof (I have had plenty of them return pitted) so I coat both my C.M. and my SS barrels. The Isonite on the outside of the stainless barrels cuts reflection down so my shooters like it.

I mentioned flaking of chrome lining inside the bore. Joel Kendrick is my contact at MMi TruTec, the company in Arkansas that does my Salt Bath Nitriding (they call it Isonite). He was mentioning using the Salt Bath Nitriding inside the bore prior to chrome lining it to get a better adhesion. He is currently working with one of the military contractors (maybe F.N., but don't quote me on that) relative to this process. He has given me permission to give out his contact information so I have cc'd him in my reply to you. He can give you the specifics of any testing and evaluation that has been done relative to this process and can give you quotes, etc. should you just be interested in the Isonite by itself as I use it.

One last quick note on chrome lining. Such barrels have the reputation of changing point of impact when heated up. I have found this to be very true. It may be due to the way the different metals (chrome molly and chrome) expand inside the bore. What ever the reason, it does happen and, again, this was verified on a state of the art machine rest. Isonite doesn't exhibit this characteristic.

I am sure you are well aware of some of the things I covered. Lacking specific questions, I just sort of used a shotgun approach which ended up being fairly long. If I left anything unanswered please feel free to get back to me. I have enabled my Spam Blocker to allow your e-mails to come in with out the automated hate responses that Earth Link sends out. I am sure Joel would also be glad to clarify the technical aspects of the Salt Bath Nitriding process. So far I am very pleased with it.

Best of luck!
Carlos


Actually Joel Kendrick is a friend of over 10years. He started Meloniting(Isonite) my barrels long before he was hired as plant manager and engineer at MMI Trutec in Paragould, Arkansas. Melonite(Isonite) is not a coating as Carlos said. In simple terms it is a case harden nitried metal treatment. Joel was able to refine the process to a lower temperature to not affect the barrel's heat treat properties.

There are gas nitriding processes now, that work as low as 850°F.

High performance automotive engine valve springs are often nitrided. Corrosion is not an issue, but the nitrogen atoms penetrate the metal crystals and leave a very high residual compressive stress - which is a good thing for fatigue life. It's a similar, or even better benefit than you get with shot peening, even though the hard layer is only .001" or so deep.

I have a Daniel Mk12, that has the forged stainless, nitrided barrel. Still working with it as far as finding best loads but it seems to be at least MOA or better, and I doubt I'll ever wear it out.

Actually they are doing Melonite in the 550-600 deg temp. 850 is above annealing temp.

So who makes good 5.56 nitrited barrels?

Get a Krieger and have it done.

AR performance makes a very accurate 223 wylde barrel that will shoot 5.56

BCM standard chrome lined barrel:



PSA CHF (FN) chrome lined barrel:



Colt 6920 all factory (shot with irons):



BCM 20" A4 chrome lined barrel (not the best representation of accuracy, shot with irons):



I was shocked that the BCM barrels kept all rounds on target. One didn't do half bad for a chrome lined, lightweight barrel.

What was the load?

Once fired LC-09
69gr SMK
24.5gr of RL-15
CCI-BR4
2.255" COAL