View Full Version : XM16E1 barrel erosion study (1966)
This is one of many historical documents available on DTIC. I found the following interesting:
"During the test, it was noted that the phosphate-coated test bolts averaged more than 10,000 rounds each and only one failed. The remainder were removed due to the conditions of the test bolt carriers . This increase in bolt life is attributed to use of phosphate coating instead of hard chromium plate, thereby avoiding a condition wherein ·the hard chromium can be fractured easily leading to rapid propagation of the crack through the hard case and· soft core of the bolt."
So, some 53 years later, is this still a valid conclusion - that Parkerizing the bolt led to increased life over chrome plating?
https://apps.dtic.mil/dtic/tr/fulltext/u2/483309.pdf
Chrome plating, like most other processes, has improved with time. So no, 53 years later I don't believe that is an accurate statement.
ViniVidivici
04-27-19, 17:51
Yeah, I'd like to see more recent data on the subject, if I were to be making a purchase based on that info.
I mean, for example, aren't 9310 bolts better than they used to be, due to better heat treatment processes?
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.
This is one of many historical documents available on DTIC. I found the following interesting:
"During the test, it was noted that the phosphate-coated test bolts averaged more than 10,000 rounds each and only one failed. The remainder were removed due to the conditions of the test bolt carriers . This increase in bolt life is attributed to use of phosphate coating instead of hard chromium plate, thereby avoiding a condition wherein ·the hard chromium can be fractured easily leading to rapid propagation of the crack through the hard case and· soft core of the bolt."
So, some 53 years later, is this still a valid conclusion - that Parkerizing the bolt led to increased life over chrome plating?
https://apps.dtic.mil/dtic/tr/fulltext/u2/483309.pdf
Chrome plating, like most other processes, has improved with time. So no, 53 years later I don't believe that is an accurate statement.
Yes, it is a true, and accurate statement, even 53 years on. Sadly, hard chromium plating is not recommended for things subject to fatigue. That's been known for a long, long time, and it is still a problem.
On the Effect of Hard Chromium Plating on the Fatigue Limit of Steel (https://www.jstage.jst.go.jp/article/jinstmet1937/7/10/7_10_444/_article)
The Effects of Chromium Plating on the Fatigue Strength of Steel (https://www.tandfonline.com/doi/abs/10.1080/00202967.1954.11869670)
Effect of Residual Stress and Microcracks in Chrome Plating Layer to fatigue Strength of Axle Shaped Parts (https://www.scientific.net/AMM.889.10)
Effect of Chromium plating on the Endurance Limit of Steels used in Aircraft (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwin2LeIrvHhAhVrc98KHRTwBl8QFjAAegQIBhAC&url=http%3A%2F%2Fnvlpubs.nist.gov%2Fnistpubs%2Fjres%2F43%2Fjresv43n2p101_A1b.pdf&usg=AOvVaw3UAhWWPMRxQng-w0Ellejr)
There is some good news, however. For bolts and bolt carriers, you still should not go with hard chromium plating, but you can use thin dense chromium (TDC) plating. TDC goes under many proprietary names, such as Electrolizing, Armoloy, TDC-1, etc.
If you want a shiny chrome bolt, go with one that was plated with TDC.
BTW, the original Armalite coating for the bolt and bolt carrier was not hard chromium plating, but the proprietary TDC process known as Electrolizing. (http://www.electrolizingofla.com/FAQ/ElectrolizingFAQ.aspx)
Lysander (or whoever knows metallurgy), do NP3 and other popular coatings/surface treatments also have this downfall?
Electroless nickel and teflon (NP3) do not have the hydrogen embrittlement problems chrome does.
So chrome-lined bores are another animal altogether?
Nope, same problem.
http://i223.photobucket.com/albums/dd278/WTAForum/HC_Chrome_Erosion_zpsmourgver.jpg
Then, nitriding avoids all these issues?
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.
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 (https://www.m4carbine.net/showthread.php?171789-Melonite-vs-Chrome-Lined-Longevity&p=2154035&highlight=#post2154035")
http://i223.photobucket.com/albums/dd278/WTAForum/M242_Chrome_v_Nitride_w_Round_Count_zpsfjr5mveg.jpg
http://i223.photobucket.com/albums/dd278/WTAForum/M242_Chrome_v_Nitride_w_Ammo_Types_zpstlfkg1ix.jpg
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.
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?
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?
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.
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.
https://fas.org/man/dod-101/sys/ac/equip/m242_25mm_gun-s.jpg
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.
https://c8.alamy.com/comp/PKF8HT/pfc-james-ormsby-assigned-to-2nd-squadron-13th-cavalry-3rd-armored-brigade-combat-team-1st-armored-division-prepares-to-replace-the-barrel-of-a-m242-bushmaster-25mm-chain-gun-on-an-m2a3-bradley-fighting-vehicle-during-gunnery-training-at-the-doa-ana-range-complex-nm-aug-3-2018-PKF8HT.jpg
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.
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? :rolleyes:
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.
Well, I was specifically referring to issue small arms, but even the 25mm has been "Enhanced" with chrome lining. Wonder why? :rolleyes:
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.
https://www.m1919tech.com/Barrels%20and%20Jackets/fig23.jpg
Great info, learned things.
vicious_cb
04-29-19, 21:16
Obviously the solution is explosive cladding. Because explosions are cool.
Obviously the solution is explosive cladding. Because explosions are cool.
With what?
The coating has to be thin enough to clad and still offer benefits. Thin coatings do little for erosion from what Ive seen.
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.
Switzerland.
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.
https://www.m1919tech.com/Barrels%20and%20Jackets/fig23.jpg
Perfect example of something being great in theory that didn't work out in real life. Also, I'd caution against applying lessons learned from 25/30mm chain guns to 5.56 small arms. It's a data point to be sure but may not be relevant.
Switzerland.
Okay, what weapon?
Pretty sure all the Swiss Sig 55x rifle barrels are nitrocarburized. All of mine have been.
With what?
The coating has to be thin enough to clad and still offer benefits. Thin coatings do little for erosion from what Ive seen.
Tantalum alloy (https://www.tandfonline.com/doi/abs/10.1080/10426914.2012.663129)
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.
Every military that issues a Glock.
PSA has this description of their "Premium" carbine barrel:
"Barrel: A proprietary blend of Hammer forged chrome molly vanadium made by FN that is referred to as "Machine Gun Steel" by virtue of its required use in FN's M249 and M240 weapons. The hammer forging process work hardens the steel, making it more durable. In addition, the chrome process for the bore allows for a lining almost twice as thick as a standard M16 for enhanced durability."
Does the double thick chrome lining have any effect on accuracy?
Every military that issues a Glock.
Long gun, not sidearm.
Pretty sure all the Swiss Sig 55x rifle barrels are nitrocarburized. All of mine have been.
Commercial models imported into the U.S.? Yep. Can't speak for the actual Swiss military models.
Commercial models imported into the U.S.? Yep. Can't speak for the actual Swiss military models.
True. However, it seems unlikely that they would create a special production of barrels just for export. And from what I read, that is not the case.
True. However, it seems unlikely that they would create a special production of barrels just for export. And from what I read, that is not the case.
Well, there's always HK and the MR556 as an example of what I'm referring to (finished here to be sure, but certainly not treated/lined at all). The genuine Deutsch ones are chrome lined.
Well, there's always HK and the MR556 as an example of what I'm referring to (finished here to be sure, but certainly not treated/lined at all). The genuine Deutsch ones are chrome lined.Good point.
Long gun, not sidearm.
I knew that, just felt like being obstinate. I was suprised, to be honest, that I couldn't find references to military nitride barrels.
Andy
PSA has this description of their "Premium" carbine barrel:
"Barrel: A proprietary blend of Hammer forged chrome molly vanadium made by FN that is referred to as "Machine Gun Steel" by virtue of its required use in FN's M249 and M240 weapons. The hammer forging process work hardens the steel, making it more durable. In addition, the chrome process for the bore allows for a lining almost twice as thick as a standard M16 for enhanced durability."
Does the double thick chrome lining have any effect on accuracy?
I have not read anywhere where the double thickness chrome hurt or helped accuracy. The best bare steel barrels will always be more accurate than the best chromed barrels. The truly high end match barrels are neither chrome lined or hammer forged.
Andy
I have not read anywhere where the double thickness chrome hurt or helped accuracy.
The true answer that nobody will like is, "It depends." There's nothing inherently wrong with thicker chrome lining, but it depends on the bore you're starting with and the consistency of the plating in the barrel.
Goog around. Someone did a head-to-head comparison of Daniel and FN barrels. The bore finish on the DDs were much smoother and the barrels shot tighter.
Not saying a chrome-lined barrel originally designed for machineguns can't be accurate.
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