Not to my knowledge. Same goes with the other manufacturers testing. Same barrels used with different coatings.
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I don't think any of the important parts on an MR556 vs a 416 are any different other than the barrel. The 416 is chrome-lined and the MR is not. That said, as I mentioned earlier I have an MR556 pistol that I had the barrel nitrided in lieu of it not being chrome-lined. Thought that nitriding would be better than no lining at all. Don't have anywhere near enough rounds through it to comment on the durability of the nitriding though (I did it because you can't find real HK416 barrels by themselves......at least I haven't seen any and God knows what they'd cost if you could find one).
I have an AR74 with a Ballistic Advantage 14.5" barrel, haven't had any problems with it. It's also impressively corrosion resistant, I put a few hundred rounds of 7N6 (the only thing I've shot in it) through and then let it sit for a week, no signs of corrosion. Then again, I haven't tried that with chrome lined.
I believe the reason that some 5.45 barrels say to avoid some types of 5.45 is that those loads use larger bullets than they should. 5.45 bore is around .005" smaller than 5.56, I imagine the bullet diameter is different by a similar amount. Someone loaded bullets for 5.56 into 5.45 cases, and an oversize bullet can clearly cause problems. Of course, those rounds should work great in the Spikes/Lothar Walther barrel that I got that has a .218-.219 bore rather than the .214 or so it's supposed to be, maybe they wouldn't keyhole by 25yd.
No problem. When I first read the article I was too was surprised at the differences in performance, I would have thought opposite, that at lower flame temperatures the nitrided surface would perform better, however the chrome barrel has marked improvement in barrel life over the nitrided barrel at that temperature regime. However, the gap in barrel life performance closes as flame temperature increases, with more modern and future powders. Also keep in mind that data was on collected on an M242 automatic gun, which fires a 25mm projectile, which is about over an inch in diameter compared to .223.
http://ww3.hdnux.com/photos/03/31/50.../0/960x540.jpg
As a follow up, some of the articles I recall reading a couple years back I can't find now.
I'm still hunting for the white paper that showcased testing results of a pre-nitrided barrel with a hard chrome plating, but struggling to recall what weapon system it was demonstrated on and what electronic database I pulled the article from as apparently I didn't save it back then. Anyhow, in my failed attempt to relocate that article, I did stumble on what our allies have been researching.
Here's an interesting document, from DSTO (Defense Science and Technology Organisation), the Australian equivalent to our AFRL/ARL, that summarizes erosion modeling and prediction techniques for estimating barrel erosion, but before the technical discussion at the end, looking at empirical and computation data, summarizes at an executive level the erosion mechanisms (chemical, thermal, and mechanical), and also the current mitigation techniques (propellant formulations, additives, and surface coatings), note again the emphasis is on large caliber weapons systems which are more costly to maintain and replace.
Here's an excerpt from that document:
Understanding and Predicting Gun Barrel Erosion
Ian A. Johnston
Weapons Systems Division
Defence Science and Technology Organisation
http://www.dtic.mil/dtic/tr/fulltext/u2/a440938.pdf
After browsing international databases, I also came across research from South Korean Department of Weapons Engineering, that evaluated erosion of plasma nitrided 5.56 barrels, in dense and porous gaseous techniques, with M193 projectiles.Quote:
3.3 Surface Coatings and Liners
Although coatings have been used to protect barrels since World War II, there has been renewed, active research in this area over the last decade [13, 16, 18, 22, 34, 52–54]. Rather than the development of new coating materials, recent work has mostly been directed at understanding the mechanisms of coating failure, performance assessment of known potential coatings, and proposed new coating application techniques. Conroy and coworkers [34] have proposed several criteria for a successful coating:
• The coating should not react with the propellant gases.
• The coating should help insulate the base material from the heat load, distribute the heating, and be resistant to thermal erosion.
• The coating must be resistant to mechanical wear from projectile passage.
• The coating must adhere well to the base material.
• The coating must have a coefficient of thermal expansion similar to that of the base material to prevent thermal stress cracking.
• The coating material and application method must be cost effective.
According to Conroy, these myriad requirements may explain the paucity of new coatings and application techniques. Electrodeposited chromium remains the most popular barrel coating in fielded guns, despite being originally developed over sixty years ago. Other coating and liner materials that are still being actively pursued as alternatives include ceramics, and refractory metals such as molybdenum, niobium, tantalum, rhenium and tungsten.
The most common commercial technique for chromium coating is aqueous electrodeposition [54], where chromium is initially deposited as chromium hydride. During deposition and the subsequent heat treatment to outgas hydrogen, residual stress causes microcracks to form in the coating [13]. Usually the cracks do not penetrate through the entire coating thickness, however, and a crack-free sublayer exists near the base material. Refinements to the process have lead to the development of low contractile (LC) chromium coatings. LC chromium coatings exhibit fewer cracks and higher strength, at the expense of reduced hardness [1, 13]. Mawella [54] reports that recent studies on pulsed electrodeposition have demonstrated that reduced cracking or crack-free coatings are possible. A number of other experimental coating methods are also cited. Physical vapour deposition, via magnetron sputtering or the use of an RF plasma discharge, can reportedly produce crack-free coatings and deposit a range of refractory metals which cannot be electrodeposited. Chemical vapour deposition, where a volatile vapour containing the coating material decomposes on the bore surface, is noted as producing highly uniform coatings. Conventional chemical vapour deposition requires high temperatures (over 1100 K) for decomposition, thus triggering phase changes in the gun steel. Mawella proposes metal-organic chemical vapour deposition (MOCVD) as more amenable to gun barrel applications, which requires temperatures of 700 K or lower. He cites firing trials where barrels coated with chromium using MOCVD showed improved erosion resistance, compared to those coated with electrodeposited chromium. A more thorough description of these and other possible coating processes is contained in Reference [55].
Through numerical modelling and vented vessel tests, Sopok has assessed the compatibility of different refractory metal coatings and propellant types [18]. Bare gun steel, and chromium, tantalum, molybdenum rhenium and niobium coatings were subjected to oxidizing, carburizing, and intermediate propellant gas environments. Erosivity was gauged by the threshold surface temperature at which erosive processes (melting, phase transformation, and reactions) initiated. In an oxidizing propellant gas environment, rhenium and niobium had the lowest threshold (corresponding to most erosion), chromium and tantalum had the highest threshold, while the thresholds for gun steel and molybdenum were intermediate. In a carburizing environment, tantalum had the highest threshold temperature, followed by similar thresholds for chromium, molybdenum, rhenium and niobium, with gun steel performing worst. Chromium was the only material not to show a variation in threshold temperature between the different environments, which may explain its popularity as a coating material. For the other materials, the significant difference in threshold temperatures between the propellant gas environments highlights the need to match propelling charge to coating type. The chemical mechanisms responsible for the variations are discussed at length by Sopok in the paper.
The high melting point, low reactivity and high hardness of coating materials render them resistant to direct thermal, chemical and mechanical erosion. The melting point of chromium (Table 1), for example, is much higher than typical bore surface temperatures [52]. Coated barrels still erode, however, and once erosion is initiated they may erode at a faster rate than uncoated barrels [54]. Much attention has recently been given to understanding the erosion process for coated barrels.
As already noted, surface microcracks are present in chromium coatings from the time of manufacture. ´e pressure and thermal cycling of firing causes the microcracks to grow deeper until reaching the substrate material, and also propagate laterally to combine and form a network [54]. The result is fragmented but contiguous coating elements still attached to the substrate, described by Cote and Rickard [13] as a series of separate, isolated islands or plates of chromium. The dimensions of these plates are of the same order as the coating depth. Conroy and coworkers contrast these microcracks with their theory of macroscopic cracks caused by stresses at the coating-substrate interface [34]. These stresses are generated by direct loading from the barrel internal pressure, and the difference in thermal expansion of coating and substrate at the interface itself. They formulate an analytical treatment to calculate the spacing of such macroscopic cracks and, subject to a number of assumptions, find that tantalum should show less cracking (a greater spacing between cracks) than chromium. It is also determined that neither chrome nor tantalum should fail by debonding from the gun steel; instead the analysis indicates that cracking and plastic strain are the most likely results of interfacial thermomechanical stress.
Once cracks in the coating have reached the substrate, the exposed gun steel begins to erode. Jets of hot combustion gases wash through the crack, recirculate, react with the substrate, and cause pitting via thermal and chemical erosion. It has been discovered that, at the interface, oxides of refractory metal coatings may seed cracking in the substrate [22]. Specifically, Conroy and coworkers calculated that tantalum engenders more rapid pit growth in the substrate compared to chromium [34].
Numerical modelling of a 20 mm gun by Heiser and coworkers [53] showed that chromium coatings lower bore surface temperature because they conduct heat to the substrate faster. Thus the temperature at the coating-gun steel interface is higher than it would have been at the identical depth for a steel-only barrel. The high temperature at the interface encourages thermochemical erosion to traverse laterally under the coating, from the initial crack site, attacking the substrate material [16]. Eventually the coating is undermined, and susceptible to removal by mechanical processes. The small plates of coating may simply lift out due to complete separation from the steel, or be removed by engagement with the projectile or spallation [52] driven by choked high pressure gas [34] (see Section 2.3). Underwood and coworkers have experimentally observed that deep, open cracks are the preferred site of plate loss [56]. However, Sopok notes that erosion in coated cannon barrels always correlates with interface degradation and substrate exposure, regardless of whether or not this actually occurs at the deepest crack sites [22]. Hordijk and Leurs have additionally observed that once erosion of a coated barrel begins, after further firings the number of exposed spots tends to stay constant, while the damaged area per spot increases [40]. While the described process is generally agreed to be the prime cause of erosion for high temperature propellants, Cote suggests that fatigue fracture of the coating due to sliding forces may be more significant for cooler propellants [13].
Methods to prevent or reduce the undermining process have been suggested. Conroy and coworkers suggest that, aer firing, storage conditions may induce oxidation of the newly exposed substrate gun steel [34]. Corrosion control through post-firing treatment of coated barrels is thus advocated as a possibility of extending barrel life. Also suggested is pre-nitriding of the steel bore before coating, to increase hardness and reduce chemical erosion at the interface once the coating is penetrated by cracks. Likewise, reducing the carbon content of the steel near the interface may decrease its susceptibility to hydrogen cracking aer the coating is breached [56]. Alternatively, a tough cobalt interlayer located between the coating and substrate may prevent cracks penetrating through to the gun steel, and has been successfully trialed in the past [1, 13]. Underwood and coworkers also suggest that interlayers may aid in preventing the exposure of the gun steel to chemical attack, as well as decrease the transfer of shear stress from coating to substrate [56].
As alternatives to refractory metals, ceramic liners have been identified as a promising technology due to very good wear and thermal resistance. The propensity of ceramics to fracture due to susceptibility to stress concentration and flaws, however, must be addressed before widespread practical use is possible [1, 57]. Grujicic and coworkers present structural reliability studies of segmented and monolithic ceramic liners using finite element analysis, and for their 25 mm barrel test case find a failure probability of once per 400 single shots [57, 58]. The primary cause of failure was identified as cracking of the ceramic liner near the barrel ends, as a result of stress due to axial thermal expansion of the steel jacket. The use of segmented liners was found to reduce failure probability by as much as 18%, by relieving tensile stress in the ceramic. Functionally graded ceramic-tometal barrel liners provide an alternative means to avoid the abrupt mismatch of thermal expansion between a ceramic and metal interface. The response of candidate functionally graded liner materials to thermal shock, conductivity, and wear tests, are reported in an initial study by Huang and coworkers [59]. As an alternative to using ceramics as liners, Kohnken describes the use of composite reinforced ceramics for the construction of entire small-calibre barrels [60]. The concept is to use a carbon fibre/resin composite as an outer wrap, to reinforce and compress a zirconia-ceramic tube from the outside.
http://www.ciar.org/ttk/mbt/papers/symp_19/LD14_323.pdf
A STUDY ON THE EROSION CHARACTERISTICS OF THE MICROPULSED PLASMA NITRIDED BARREL OF A RIFLE
Dong-Yoon Chung, Hee Jai Kim, H. N. Kim
Department of Weapons Engineering, Korea Military Academy, Seoul, South Korea
19th International Symposium of Ballistics, 7–11 May 2001, Interlaken, Switzerland
Here's an excerp from the end of that document:
What I'm unsure about is whether the first test article, "A", used as a base line comparison was a barrel in the white, or chrome lined. It wasn't explicitly stated, the only description was the "barrel material used in the present". I assume that it must therefore be CMV at the nominal, but unknown if it was plated, based on the pictures, that don't show a clear delineated chrome lining from the cross sectional views, I assume it was in the white.Quote:
4. CONCLUSIONS
The surface of wear sensor was put under nitriding and postoxidation treatment after controlling the compound layer densely or porously using the micropulsed plasma nitriding technology. The sensor was inserted into the free flight zone and the center zone. The
erosion characteristics by the propellant gas were measured and the following conclusions were obtained.
1) In the free flight zone, oxidation after porous nitriding treatment shows the best antierosion characteristics and the preservation status of the compound layer.
2) In the center zone of the barrel, there are no big differences in the anti-erosion characteristics for the different surface treatments. However, there is peeling of the oxide layer by tangential cracks under the surface resulting from shear stress.
3) Comparing the environment between the free flight zone and the center zone of the barrel, the pressure from the propellant gas is the dominant factor in determining the erosion characteristics.
4) Regardless of the surface treatment, the wear increases linearly as a function of the rounds fired.
5) The peeling of the oxide layer which occurs in the center zone of the barrel, needs to be studied further for validating the micropulsed plasma nitriding technology.
Below are some pertinent figures from the report, A - baseline (again unknown if chromelined or in the white, strongly leaning in the white), DNO is the designation for densely nitrided/oxidation, and PNO for porous nitride/oxidation. The research was only conducted on those three test articles, barrels, with a "sensor" section, essentially a coupon cut out near the chamber at the free bore end (labeled FFZ - free float zone), and at the center of the barrel, designed CZ - center zone, in the report. Unsurprisingly, the erosion and crack formation is highest near the chamber. However, interestingly, no noteworthy cracks were observed at the center, unknown barrel length, for the baseline barrel, but both nitrided barrels showed tangential cracks on the surface. Based on this preliminary set of data, it would seem that the porous plasma based nitriding techniques is superior than the dense processing techniques with gaseous nitriding, but unsure how that translates to other methods currently employed like salt-bath.
http://i223.photobucket.com/albums/d...psdipelqsk.jpg
http://i223.photobucket.com/albums/d...psv7bdqeec.jpg
http://i223.photobucket.com/albums/d...ps5ohwi303.jpg
It's the most relevant research I've found so far to small arms evaluation of chrome v. nitride, as most if not all seems to be focused on large calibers. I wished the author would have explicitly stated if the baseline barrel was in the white or chromelined, would have evaluated alternative nitriding techniques like salt bath, and would have sent more rounds down range, up to the end of barrel life to the TDP nominal precision/MOA requirement for performance.
If I can find other data, or follow up research I'll definitely share it here. Note, everything I've posted thus far is approved for public release, for those that maybe concern with proprietary data, ITAR/OPSEC issues.
I have a few AR's with Melonite barrels and this is what is stated under each barrel the manufacturer I purchased them from sells:
**Melonite treated barrels have proven to last 30% longer than chromelined barrels and are more accurate. Hi perf. chamber and barrel allow for ANY factory ammo to be used or gain more with handloads.**
I know everyone whom uses their barrels praises them for their accuracy as Match barrels but the manufacturer does not claim them to be match barrels. Most people are claiming to get 1/2" groups or better at 100 yards.
To be a little fair, all my barrels are 6.8SPC and are very accurate but he makes plenty of 5.56/223 barrels people worship.
I can only attest to their 6.8 barrels.
It would be unusual to compare this in a way such as this. A sales department could use similar tones, as I would expect them to do so. I would not be hesitant to see the processes of either of the samples in that contrast,.The end result is how well it can perform in the duration of it"s lifecycle for it"s own intended purpose.
For a large majority of shooters, barrel longevity never matters. It wouldn't surprise me if an average AR goes through less than a few hundred rounds a year. Similarly, most people won't notice the difference between a barrel that'll shoot 1/2MOA and 1MOA, as they won't use the right ammo to take advantage of that - they'll shoot cheap crap usually, sometimes splurging on M855. However, the lower price (vs chrome lining) and corrosion resistance of nitrided components are great for typical shooters who likely don't take care of their guns. I just wish that there were nitrided wheelguns readily available, I think it's a great alternative to bluing.
That could just be in your particular experience.
Many people who review the M&P15 Sport with the Melonite barrels and run steel-core/case ammo don't have issues with it. I've owned a Sport model myself and had no problems running a few hundred rounds of Tula223 through it with no issues, but that was not nearly close to putting high round count on it to confirm.
A good test would be if someone can compare both a good CL barrel (e.g. BCM or DD) and a good Nitride barrel and run 5-10k rounds of ammo through each in semi over a period of time.