How Well Does Direct Impingement Handle Heat?

DI is usually identified as the weak link in the AR-15 system when it comes to sustained fully automatic fire. How vulnerable is that system to fully automatic fire? That’s a good question, but fortunately we have a few resources with which to examine it.

To start, we’ll look at Iraqveteran8888’s test of a customized upper receiver mated to an M16A1 lower. We can see in this test that, despite the brutal firing schedule including four 50-round drum dumps, it is the barrel that fails first, before the DI gas system:

The next piece of evidence are two internal Colt tests, first with an M4 and then the heavier-barreled M4A1. The videos from this article are well worth the watch, but do not embed, so please head on over there and watch both of them first before continuing to read. The M4A1 video, fortunately, is mirrored on YouTube, and is embedded below:

In the Colt tests, the M4, like IV888’s AR-15, bursts its barrel first before the gas tube fails. The M4A1, however, with its heavier contour barrel, bursts its gas tube first and becomes a repeater.

What’s clear is that in many cases, despite becoming glowing hot, the gas tube will outlast the barrel itself. Even in the Colt M4A1 test, the barrel has become shot out and useless before the gas tube goes (the barrel noticeably begins to droop at around 4:15, and the gas tube fails soon after). Further, in all these tests, none of the rear operating components (bolt, carrier, receiver), fail. The rifles could, conceivably, be rebarreled and put back into service. In general, two things can cause a gas tube to fail earlier than the barrel in fully automatic torture tests like these: Having a heavier barrel profile with more thermal mass (or perhaps, an aluminum heat sink, like the Colt and DSA IARs), and repeatedly using higher-capacity drum magazines.

I’ve linked to it before, but Hognose has written an excellent overview of the M4 Carbine and its problems with heat – which are mostly shared with all air-cooled, closed-bolt weapons. The essentials of his post are excerpted below, but I highly recommend our readers click through and read the whole thing:

Bad Things Happen When Barrels Get Hot

The peak temperature area in the barrel is usually about three to seven inches forward of the chamber, depending on caliber (according to the references, on 5.56 mm rifles, it’s about four inches). This is where the thermal stress is at peak, and it also has to support all the rest of the barrel (and anything that may be attached to it, from a Surefire to an M9 bayonet), so when the gun is going to fail, it’s probably going to fail near here.

As more rounds are fired, more heat builds up, because it is being added at a higher rate than it can be radiated away. As the temperature rises, bad things happen:

  • You have a risk of propellant cook-off. Weapons that fire from closed-bolt are especially prone to cook-off. At a critical temperature, the powder or primer will self-initiate. As the temperature rises, the amount of time a round has to sit in the chamber to heat-soak to the point that it self-initiates declines. At first it takes minutes, then seconds, then rounds can actually cook off before the automatic firing train fires them, and finally, they can cook off out of battery. Usually other damage disables the weapon by this point.

  • It can cause failures to extract.

  • It can cause the barrel itself to fail next time it is used. At a very high temperature, the barrel is heated until it loses its temper, which can cause an invisible (and undetectable by gaging) failure of accuracy. This was first noted with aerial machine guns in WWII, as we noted here before.
  • If continued, it can cause the barrel to fail catastrophically whilst firing. Stripped of its heat treatment and heated to the metal’s plastic temperature, the barrel droops. At first, rounds extending through it will sort of “hold it up” but soon it will be unable to contain the pressure and will burst.
  • If the barrel doesn’t fail first, heat can cause the gas tube to fail. Weakened by high temps, the tube lets go.

Any gun can cook off. The USN famously cooked off a 5″ on the destroyer USS Turner Joy in 1965 during a Vietnam War shore bombardment, killing three sailors and wounding three more.

How to Deal With Heat Limits

The Training Answer: First, every GI should see those Colt test videos and know what his gun can, and can’t, do. While the Black Hills guys were correct in noting that SF/SOF guys usually manually fire single shots or short bursts, even most of them don’t know what happens when a gun goes cyclic for minutes at a time. A good video explaining “why you can’t do that” would be a strong addition to training, not only for combat forces, but for support elements who may find themselves in combat and feel the urge to dump mags at cyclic rate.

The Morale Answer: Every GI should see the same done to AKs as well. There is a myth perpetuated by pig-ignorant people (like General Scales) that the AK series possesses magical properties and that the American weapons are crap. In fact, nobody I know of at the sharp end is at all eager to change, perhaps because the laws of physics and the properties of materials apply just as firmly to a gun originally created by a Communist in Izhevsk as they do to a concept crafted by capitalists in California. If you’ve ever fired an AK to destruction, you know that it grows too hot to hold, then the wooden furniture goes on fire, then, if you persist on firing it full-auto, it also goes kablooey. Not because there’s anything wrong with this rifle, but the laws and equations work the same for engineers worldwide.

The Systems Answer:  As you can see from the Colt videos, if you clicked on over to Chivers’s article, thickening the barrel nearly doubled the rounds to catastrophic failure on cyclic. An open/closed bolt cycle might have practical benefits. They wouldn’t show up in sustained heavy firing like the destruction tests, but they might show up in how a weapon recoups from high temps, and open-bolt autofire would eliminate cook-offs, at least. But any such approach needs thorough testing.

The Wrong Answer: Replacing the M4 with something like the SCAR or the HK416, something that is, at best, barely better, that is much more maintenance intensive, and that, contra Scales’s assertion that his undisclosed client’s weapon is “the same price,” is twice (SCAR) or three times (416) the money. (The 416 mags are the best part of the system, though).

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at


  • Riot

    “much more maintenance intensive” – What a crock of horseshit, DI guns require more intensive and frequent maintenance, scrubbing carbon isn’t nothing.

    • CommonSense23

      How do DI guns require more maintenance?

      • Joshua

        Because he thinks white gloving his gun actually makes it more reliable, he just doesn’t realize it was done to keep him busy.

        • Yellow Devil


          • Yeah, unfortunately.

          • Andrew

            Mr Merrill at MilCopp Tactical has a good article on cleaning and maintenance. Put the brake cleaner down. Back away slowly.

      • Riot

        Cleaning is maintenance and there is a larger area thats needs proper attention, which means breaking the weapon down more often.

        • Joshua

          Useless scrubbing designed to keep privates busy is not maintenance, it’s busy work plain and simple.

          When I was deployed in the field you can literally take out the BCG wipe it down, wipe down the upper, and run a bore snake down the bore(they use shoe strings dipped in oil over there), relube and be done.

          A more serious cleaning where you actually take down the BCG is not needed daily.

          Seriously if a brush can’t reach it, oil will.

        • CommonSense23

          Areas that don’t really matter.

      • DIR911911 .

        they don’t. the time you would spend on a D.I. system cleaning the bolt where a lot of the carbon deposits is changed to cleaning the piston where the carbon builds up on a piston system rifle. same carbon , just builds up in a different place on the gun. since this isn’t where the guns are failing it kind of makes it a mute point though..there’s alo a video on youtube showing both types of guns being shot and filmed with a thermal image camera and you can really see how the heat travels back into the bolt and rest of the upper on a DI system, while the heat seems to stay better isolated to the barrel and piston system on the piston gun.

        • Andrew

          Dude if you overheat your bolt, you have trashed the barrel first.

          • Andrew

            I’m agreeing with dir9119. I suck at wording.

      • Georgiaboy61

        The direct-gas impingement system on an M16/AR15/M4-style rifle or carbine cycles hot propellant gases and combustion products from the barrel (behind the fired bullet) directly back into the bolt/bolt-carrier group – rather than diverting and exhausting the fouling and hot gases through some other means. DGI systems can be effective and reliable but you do need to stay on top of preventive maintenance – just as you would with any mechanical device.

        • CommonSense23

          But what parts on a DI system are going to need replacing before someone who runs a op rod system. Cause after years of running a DI gun, and almost always suppressed. And routinely going between 2000-3000 rounds without cleaning or malfunctions, I keep getting confused by people telling me my gun needed more maintenance than a piston.

          • Georgiaboy61

            Re: “And routinely going between 2000-3000 rounds without cleaning or malfunctions, I keep getting confused by people telling me my gun needed more maintenance than a piston.”

            Sounds like you have more practical experience than I do – and are better-qualified to weigh the merits (or lack of same) of the DGI system… but please note I didn’t say anything about piston/short-stroke piston designs in my original post. Deciding which of these designs is “better” is above my pay grade! I’ll leave that to the pros.

        • Right, I agree: AR-15s are mechanical devices (as are piston guns) and to expect them to run forever without proper maintenance is folly.

    • Vitsaus

      People also don’t realize that soft, hot carbon is not as unforgiving as cooled hardened carbon on gun parts. Dump a couple hundred rounds and let the bolt cool. Sometimes troops in the field don’t have time to sit and clean guns between fire fights.

      • CommonSense23

        I am curious how that would matter if the carbon would cool? I would go thousands of rounds without cleaning or malfunction on my rifle and they were all suppressed.

      • There are only two places you’ll find hardened carbon on an AR-15: The flash hider and the bolt stem.

      • Uniform223


    • Maintenance is not the same as cleaning.

      • CommonSense23

        One day hopefully people would understand that maintenance is meant to be the replacement of parts before they fail. Not after.

        • WFA

          “Preventative maintenance” is the term used widely in industry to ensure people understand why it is necessary.

  • Vitsaus

    Looks like we can replace all those 249s with M4s now.

    • The USMC basically did.

      • Joshua

        My god don’t get me started………don’t do it.

        • Uniform223

          The USMC has both a piston HK AR-15 design (M27 IAR) to be used along side the SAW. I don’t see the point in it either…

  • Brian M

    “The Wrong Answer: Replacing the M4 with something like the SCAR or the HK416, something that is, at best, barely better, that is much more maintenance intensive”

    Wait wait wait! Wait up, I thought that the SCAR and 416 were the new greatest thing since Picatinny rails because they were notably superior the M4 and were far more reliable while needing far less maintenance, or so I thought, according to what you guys were posting as recently as the end of last year, which was just a couple months ago. If the M4 is so blameless and sublime in all ways, then why have all you guys been so strong in admonishing it, or at least so quiet in supporting it throughout all these years, but now you’re all suddenly now dyed in the wool M4 supporters. Where were you guys during the Vietnam debacle? Where were you with the complaints in Somalia? Where were you when the 90’s advanced weapons program wanted to supplant the M16 Where were you when the M16’s and M4’s failed in Iraqi Freedom and it was documented that guys were really picking up alternative weapons? Where were you when the 2007 dust test broke? Where were you following Wanat? Where were you following the years long M4 carbine controversy? Where were you when the M4 got whipped again in the most recent competition so hard that the Army suddenly pulled the plug on the whole thing?

    A weapon system cannot be plagued by strong criticisms throughout its entire life, be widely acknowledged by trustworthy sources as inferior to new rivals, have this carry on for years, and then that same first system, without any changes at all, suddenly become indisputably equal to or better than those things which were universally praised as being the next leap forward, especially when they haven’t changed, either. No, no I don’t believe that. Something’s up. Either the 416 and SCAR are not suddenly the wonderful next generation tools which you have been praising loudly for years, or the M4 is suddenly the greatest thing ever, according to you, the same people who have been saying that the M4 is indisputably outdone by the SCAR and 416. It’s either that, or you have something to gain by flocking to a new darling, or you have been deliberately dishonest with the real state of things. Which one is it? Tell us, we are curious.

    • Joshua

      But there have been changes.

      What you forget is that the HKM4 program came about in 2003 as a way to get a rifle the size of the MP5. Then the only thing available was the Car-15. The Mk18 was still early phase of testing. The HK416 was never purchased to replace the M4, but to get a SBR faster than SOCOM could. Fast forward now and the only two units using it are the two who adopted it in 2004, everyone else is using the Mk18 or CQBR.

      The SCAR was a whole nother matter. It also began in 2004, but eventually morphed into a multical rifle that could be both 5.56 and 7.62 with minimal parts change. It took a long time to get the conversion kits finished so we had the Mk16 and Mk17. The issue is by the time these programs really finished the M4A1 had multiple ECP’s that passed Army testing, and the SOPMOD block II was finishing up.

      If you compared a 2003 M4 to a HK416 or a MK16, yeah they are miles better. The issue is when you compare a modern SOCOM M4A1 to them the advantages are no longer there, and best of all the M4A1 can be had for around $1,000 total(after the RIS II and SSF are added). That is 1.5-2x cheaper for a rifle that does everything as well.

      • Brian M

        Good response. I’ve done some digging around, and I have to refute your arguments on a few points.

        The first is that the M4a1 is somehow a super new upgrade to the M4. The M4a1 has been around since around 2009, if I remember correctly. 2009 was the year also that SCAR and 416 starting catching on, and the past six years have had the general consensus be that they are superior to the M4, even the M4a1. In fact, they outperformed the M4a1 in the 2014 tests just a few months ago. And this test was done with choice rifles from all sides, though the 2007 results pertaining to the rack grade lightly used M4’s ought to horrify anyone.

        The second is the idea that the 416 and SCAR have been dropped widely. The truth is that they’re in fairly common use with NATO and similarly-minded countries, and are becoming more popular and widespread with every passing day.

        Thirdly, I have no idea where the idea of the 416 existing as an attempt at making a CQB carbine before anyone else could, because the M4 already existed more than a decade before the 416. And before the M4, there was the CAR-15. The 416 started life, as far as I know, as a concept for an improved CQB carbine with enhanced reliability.

        The fourth is the truth about what the M4a1 is, mechanically. The M4a1 is just an M4 with a stiffer buffer and a heavier barrel. That’s it. The results showed in 2014 testing — better than old M4, but not a wonder rifle, or up to par versus its rivals.

        • CommonSense23

          I am curious where you get the idea that the SCAR is even close to superior to a M4A1? Or that that the 416 is a superior rifle. Cause every time one of the SF groups does a comparison it comes back the same. No need to change from the M4A1/MK18 to the HK416. SOCOM also agreed.

          How about those 2007 and 2014 test. The ones where the army specifically doesn’t use PMAGs. Even though they have been proven to be more reliable.

          And the idea that the 416 was designed primarily as SBR in the 10inch barrel range that was meant to be used suppressed is pretty widely known.

          And if you believe the only difference between a M4 and M4A1 is the buffer and heavier barrel. You really need to see to educate yourself on the improvements it’s had.

          And with your earlier comment, you rehash so many earlier complaints that have already been deconstructed.

          • iksnilol

            If the 416 is so bad then why is Norway using it? Norway can have whatever it wants and they wanted the 416? Why is that?

            Note: I ain’t got no dog in this fight. I prefer Eastern weapons, more comfortable for me.

          • Joshua

            Its not a bad gun, its just not three times the gun the M4A1 is. The barrel is the only thing truly better on the HK416 due to materials used and the tapered bore.

          • CommonSense23

            And ironically enough, the barrel being far superior is why SOCOM is one of the biggest reasons the SCAR program survived.

          • CommonSense23

            It’s not bad at all just expensive. It’s biggest strength is it has a better barrel then the M4. That is about it.

          • Tom

            Whilst I doubt the pressure to purchase from within the EEC was the deciding factor I think it must of been up there. Which basically left very few mature designs to choose from being the HK 416 and G36 along with the FN SCAR or FN2000. An existing relationship with HK probable helped things along as well.

            As far as I can tell the 416 is superior* to the DI guns if you want to run a suppressor but that’s about all. Its certainly a lot more expensive.

            Also I recall Norway had some problems with their rifles which was blamed on ammo.

            * I say superior as in its better not that you can on an 416 and not on an M4 if that makes any sense.

          • iksnilol

            *you mean it has an advantage because it can do something the M4 can’t? It is perfectly understandable.

            Like I said, I have no dog in this fight. I am a civvy, use whatever I can get my hands on (preferably AKs though). Can’t wait til the day I get a Dragunov (the civvy version, a Tigr). Put a Lothar Walther barrel on it, telescopic titanium suppressor, and a custom folding stock. Won’t be cheap but won’t be expensive either. Hardest thing is going to be adapting it to use M1A/M14 mags (getting the 308 version obviously). That is one of my dream rifles, and it is also one of the realistic ones (and it is gonna be handy in Norway).

          • CommonSense23

            He is wrong on that regard with Suppressed shooting. Current DI designs have no problem with suppressors these days.

          • iksnilol

            But don’t they get super dirty then? I know DI gets really dirty.

          • Joshua

            They all get super dirty. Suppressors send a ton of fouling back down the bore after the case is extracted. There is little difference in fouling when comparing a CQBR and HK416 suppressed.

          • Andrew

            Joshua is correct.

          • Uniform223

            Doesn’t the Mk18 have a single piece gas ring compared to the 3 piece gas rings for the M4/A1s?

          • Joshua

            No. It was tested but showed no significant improvement in durability or reliability.

          • Tom

            As I recall (and I have no idea why I am thinking this because to be honest I do not even think it makes the slightest sense from a mechanical point of view) that you need a different buffer for reliable suppressed use and hence its not optimal if you want to run with a suppressor then take it off and put it back on without changing other things.

          • CommonSense23

            Well not simply true. Was issued a suppressor the majority of my career. Never had to do anything special for it. The DI systems, M4A1 and MK18 just put on/takeoff and play, no adjustment needed. The piston guns I all ways had to adjust the piston.

          • Tom

            Makes absolute sense its a self regulating system after all. I think maybe I was unwittingly reciting some marketing properganda from a well known German tool manufacturer and its fan base.

          • Uniform223

            As I understand it to make the DI carbine length gas tube more compatible with a suppressor had nothing to do with the tube itself but the size of the gas port. Also as I understand it what counts more in having a suppressed firearm especially a rifle or carbine is how is the gas regulated. That is where piston systems seem to have an edge as excess gas or pressure caused by the suppressor is regulated better at the gas block.

          • CommonSense23

            Your would have been kinda right a decade ago. And mainly with the SBRs. Again why the HK416 was developed to begin with was to solve some of the problems with a DI gas system and suppressors. But with the work Crane did. And the understanding we have now of the DI systems, coupled with you can get a adjustable gas block for DI guns has pretty much erased any advantage a traditional gas piston had. And couple that with some prototype suppressors that offer almost zero blowback.

          • Tom

            I meant to say that the you can run a suppressor on the M4 its just not as reliable as on the 416.

          • Andrew

            The 416 is not a bad gun. But its not magic, either.

          • Uniform223

            I would like it more if it wasn’t so expensive and heavier. I personally do not see any real trade off or “benefits” to warrant it.

          • Andrew

            My point exactly, sir.

        • Brian, there’s a big difference between the M4A1 of today and the M4A1 of 2001. They don’t even have the same barrel contour.

        • Lance

          Strange the SCAR was so great to you. Then SOCOM ditched the L version and cut number of H being bought for more M-4A1s with SOPMOD kits. Seem your SCAR loving emotions get in the way of facts.

          • Brian M

            I have nothing vested either for or against any weapons in this fight. I’m just wondering where from the massive outpouring of pro-M4 sentiment came from all of a sudden.

        • Joshua

          The ICC results came out and showed that one rifle performed better in class I and II stoppages, and that the M4A1 performed the best in Class III which is the most important category. And it was only one rifle to perform better in one category.

          What other countries do does not affect us, and other countries to not have the SOPMOD program nor the COTS options available, nor the civilian market manuf. available that we do, this greatly opens our options to fix shortcomings in our rifles.

          Before the HK416 was the HK416, it was the HKM4, a op rod drop in conversion for the M4A1. It came about because SFOD-D wanted a 10″ sub gun with 5.56 ballistics, and the CAR-15 was anything but reliable. Around this same time CRANE was working with Colt on the Mk18, but it was still years away from being reliable. Thus the HK416 was born and it was ahead of its time in 2004.

          In 2007 Colt got new springs and improved buffers approved that greatly increased reliability, then SOCOM had the SOPMOD Block II items finalized in 2009 that added even more capability to the M4A1. So yes it is a very different weapon.

          As for the ICC…..ONE rifle outperformed the M4A1 in Class I and II stoppages and NO rifles outperformed it,in the more important Class III stoppages where the M4A1 performed the best.

          It is also worth noting that the ICC brought to light major problems with the tan follower mags and M855A1. Revisions are due soon that change front wall height and feed lip alignment and angle on GI mags that increase reliability by 300%. I will also point out 80-90% of the stoppages for the M4A1 were magazine related…so you do the math there.

        • Andrew

          The M4 A1 is definitely older than 2009. I’ve been using one for 11 years. And it is a good rifle. And yeah, I’ve shot it on auto too, more for fun than training value. I’ve even shot it suppressed for many hundreds of rounds, gave it a 15 minute wipedown and some lube, and shot it some more. I’ve taken it swimming many times both by accident and on purpose. The gun is simply not made of glass. Keep an accurate round count and replace wear items like gas rings on schedule and it will treat you right. I even own a near exact copy of my work gun in my personal collection. Only real difference is semi trigger and failzero carrier, not because its more reliable, but because its absurdly easy to clean, even though I don’t. I have not taken the bolt out of the carrier since I last replaced the gas rings. Thousands of rounds ago. That gun has had literally 1 fte ever. With a reloaded round I found on the ground. Sorry for the long rant, but I am f@&())g tired of people bashing the tool of my trade because their armorer is low budget or lazy. Put your dental picks and q tips away and go train.

        • Andrew

          416 is also heavier and more expensive. And no the mags aren’t super special. I have a dozen of them that were given to us with no expectation of return. I used them exclusively during the trial period and switched back to gray mags. Then I was given gray mags with magpul followers. Then Pmags. The black mags don’t do anything better in my experience and weigh more. In fact they’re doing a great job of holding down a shelf in my shed, waiting for the next awb so i can sell them to people with more money than bullets.

          • whskee

            I had the same experience when we got a pallet of those mags. Anyone who wanted them got them, and nearly everyone went right back to PMAGs after the first field use. Simply too heavy for too little benefit.

        • Andrew

          And sorry, Brian, I don’t mean to call you out personally, just hit reply.

        • Bill Butler, MSG USA ret

          You “refute” but you prove nothing with references or published facts. Your statements are no more than your opinion, without proof. Please provide references and proof of your statements.

          • Dolf

            His post did contain facts. Yours is griping that he didn’t pump out 50,000 words in an academic paper blessed by Larry Vickers, Samuel Colt, Dr. Robert Fackler, and Taylor Anderson.

          • Andrew

            It is mostly mostly my opinion. I also expend more ammo in a month than most MSG’s have expended in their entire career. Of course, this is a comments section on the internet, so I can make any claims I want. I won’t argue, but I will encourage you to do some side by side evaluations of your own. And if you decide the 416 is better, thats cool with me.

    • Wanat.

      2007 dust tests.

      The fleet yaw problem (related to Somalia).

      The 1990s ACR program.

      The M4 ranked 2nd, 2nd, and 1st for each class of malfunction (the last class being the most serious, and the M4 being the best performing) out of four rifles. So it was not by any means “whipped”.

      Have you considered that some of the trustworthy sources you mention may have been trying to sell you something?

      I have nothing to gain by saying good or bad things about any rifle. I am just a researcher, and whatever opinions I have are influenced by the data that are out there.

      • Joshua

        Technically it was 80 rifles….10 of each bid and 8 bids.


        • I thought by Phase II they had only four.

          Please email me Joshua, I would like to start a private conversation.

      • Brian M

        Thanks for responding, Nathaniel. Not implying that anyone’s got an agenda — I was just curious why it’s taken people so long to start arguing in favor of the M4 carbine, say around now, instead of say, immediately following the 07 tests, Wanat, and the 2013 competition cancellation. It strikes me as being a bit like trying to vote on November 15’th, the history’s already been written and people have their minds made up, if you know what I mean.

        • Well, I don’t know, as I wasn’t blogging in 2007. In what way is history already written? The US Army just adopted the M4A1, not a totally new rifle or caliber.

    • n0truscotsman

      Others have responded to the technical side of things, so Ill address one other thing as it would be unnecessarily redundant.

      “If the M4 is so blameless and sublime in all ways, then why have all you guys been so strong in admonishing it, or at least so quiet in supporting it throughout all these years, but now you’re all suddenly now dyed in the wool M4 supporters.”

      Many of us have been M4 supporters since M4s hit big army in standard service. I’ve been a supporter of Stoner’s tech since I grew up with my dad’s SP1. Mind you, this was in a era where SP1s and the like were niche specific and the myths were still rampant about M16 unreliability from Vietnam.

      It also didn’t help that there was also a mythology behind the M14 before now. Supposedly, by the supporters hyperbole, this rifle was the hammer of Thor itself.

      The reason why you perceive our newfound “love” for the M4 is because 2 things:

      1.) The recent IICC tests proved the M4 to be, overall, the superior rifle. Ending the 2007 dust test talking point frequently thrown around by critics.

      2.) The supporters of the other carbines, who were like sharks in a bloody pool following Wanat and the Congressional inquiry, are mostly drinking a large cup of STFU.

      And, thanks to the efforts of countless knowledgable people out there over the past 5 years, the enormous amounts of information effectively refuting the “unreliability” myths of the Stoner rifle and numerous personal anecdotes that are taken as facts by detractors.

  • Nathaniel, there is an M-16 versus Galil test that the Israelis performed in the late eighties/early 90s that you might enjoy. It is on YouTube somewhere and shows the M-16 cooking off time and time again while the Galil is sitting pretty.

    • Joshua

      Yes, but by moving from double,shield handguards to the KAC RAS we went from 170 rounds before cookoff to 210. By increasing to the RO921HB barrel and the DD RIS II we went to 240 rounds before cookoff.

      The double shield handguards may reduce heat to the hand, but they greatly increase heat retention at the barrel.

      • That I do not understand. How on earth do the handguards affect cookoff temperature? Are they acting as radiators or something?

        • CommonSense23

          I am guessing the same way we were melting our EOTechs with the Surefire Suppressors.

        • Joshua

          In a way. Polymer handguard with heat shields keeps heat trapped inside to protect the users hands. This leads to higher barrel temps and slower cooling.

          Aluminum handguards with steel barrel nuts absorb the heat from the steel nut and transfer it to the handguard. They are often also better ventilated. The RAS has the issue of bottom heat shields which blocks multiple vents.

          The faster you transfer heat from the chamber are of the barrel to the handguard and cool it, the longer chamber temps stay below cookoff.

          This is one upside to the handguard on the HK416 which is a big beefy aluminum rail, with a big beefy barrel nut. To draw heat from the chamber to the handguard.

          Same with the Geissele rail on a AR.

          • Very interesting. I hadn’t been considering any factors besides the barrel and possibly also headspace. I wonder if the steel body of the Galil is sucking heat off the trunnion and thus chamber?

          • Joshua

            Barrel thickness also plays a role, which is why the M4A1 has a higher round before cookoff than the M4 if all variables are the same.

          • Right, and apparently so does barrel length. Bizarrely, this apparently works in favor of shorter barrels.

    • hami

      Now THAT is a salesman. He has that way about him. He is completely comfortable with the platform and walks that fine line between professionalism and showmanship.

    • Vitsaus

      Obviously the cook off was the result of CGI or slight of hand. Perhaps astral projection was employed to move the trigger across multiple dimensions. The AR15 is incapable of being out performed in any regard by any other platform at any time, under any circumstances.

      • That’s a very silly thing to say!

        • iksnilol

          I think he was being sarcastic.

          • That doesn’t mean it wasn’t a silly thing to say.

          • iksnilol

            But he has a point. Everybody is like “ARs fantastic! Nothing wrong with them. If it isn’t an AR I don’t want to hear about it. etc.”

            Then when there shows up things that prove that they go “WOO! We were right” then when they are shown something that disproves that they go defensive.

            We still have to remember the subjective nature of firearms.

          • You know as well as I do that there are way fewer mindless fanboys of the bone stock Colt M4 out there than the M14, HK 416, SCAR, etc.

            And besides, given his commenting history I think it’s clear that Vitsaus is attempting to make fun of me, in particular, which is very silly considering I am just collating data. If the data don’t agree with him, he should go to their source.

          • iksnilol

            Eh, I have seen it go both ways. + I always thought the SCAR, ACR, whatevs fanboys were mall ninjas and thusly I dismissed them.

            You see Nathaniel-sensei, I am of old school, when fate gives me FA AK I say “thank you”. When fate give me Krag Jørgensen that has been converted to a single shot .22 (collectors cringe, it was made in 18XX, I believe it was 1888 or 1898) I again, say “thank you”. Honestly, as long as it has a trigger and a projectile I can make it work. That is pretty much my motto in regards to weapons.

            Though to make a soldier gun, your best bet is to get conscripts (not leet operators or professionals) and have them test out different mechanisms and controls and see what they like the best. For instance, I talked to a decent deal of people who were/are in the Bosnian millitary. In case you don’t know, the Bosnian millitary is a total Charlie Foxtrot (in millitary terms) in regards to logistics (we got AKs and M16s side by side for chrissakes).

            And what many of those men and women told me was, they liked the AR because it was so accurate (accurate meaning that the sights were easily adjustable) while the AK sights aren’t as easily adjustable. So that simple, not seemingly important detail was one of the things they remembered the most about the two weapons. But they didn’t like how the AR got dirty so quick (necessitating cleaning or else you can have a bad day, especially in the mud). Sure, there were also other things like liking 5.56 due to how light it is while liking 7.62×39 for its effectivenes in shooting through enemy cover. How heavy is the 6.5mm Grendel in comparison to 5.56 and 7.62×39?

            Sorry for wall of text.

          • 6.5 Grendel is about the same weight as 7.62×39, depending on the bullet weight. 5.56mm is about three-quarters the weight of either.

          • iksnilol

            So not much difference, but you get improved range so there is that. Personally I have no problem with weight of 7.62×39 but then again I don’t wear body armor or carry a bunch of equipment.

          • An inconvenient fact for the larger-caliber crowd is that the Soviets thought it was absolutely militarily significant that 5.45mm used less resources both in its construction and in its delivery to troops that they made an expedient change-over from 7.62×39 to it, even fielding the AK-74 before NATO adopted the 5.56mm.

          • iksnilol

            They also thought that invading Afghanistan was a good idea. Ruskies are smart, but even they make blunders. If it wasn’t a blunder, they wouldn’t be using 7.62×39 AKs unofficially and guard the few AKMs in inventory (for millitary) as if they were the holy grail.

            But I will admit, 5.56 and the like isn’t bad for a millitary that carries a lot of heavy equipment (read: every modern millitary) and needs to save weight wherever it can. I prefer the versatility of 7.62×39 and its efficency (though stay away from M43, it is just bad). Y’know, if it ain’t broken don’t fix it.

            Though I do find it funny how 5.56 and the M16 was first made for full auto spray and pray, then later on it is rebranded as a pseudo sniper rifle. I remember seeing a Vietnam era manual that recommended practicing flicking the selector to full auto in one motion (to break it in and make it muscle memory), “because you are going to need all the firepower you can get in an ambush”. Almost a direct quote.

            On the contrary the AK selector is made to hit semi auto during an adrenaline filled situation where fine motor skills are impaired. That’s the reason why FA is before SA on an AK, you are meant to slam down the selector going past the FA setting and stopping on the SA.

          • Russian small arms designers invaded Afghanistan? When? Please, gimme a break. Yes, Russians are human, no that fact does not invalidate their adoption of 5.45×39. Sheesh! Will you make any argument to try to discredit SCHV?

            Russians do not use 7.62×39 AKs unofficially, and I have heard no reports they they “guard the few AKMs in inventory as if they were the holy grail”. That doesn’t even make sense; if they wanted 7.62×39 AKs, they can buy them from Izhmash! Heck, if they wanted 7.62×39, why did they trial both the A545 and AK-12 in 5.45×39? I agree that 7.62×39 persists in certain roles, but that is a far cry from conclusive proof that the adoption of the 5.45×39 was a “blunder”!

            7.62×39 is not “more versatile” unless you live in American state with caliber restrictions for medium game hunting. SCHV rounds will do everything 7.62×39 will do, and outrange that caliber by 100-150m from comparable barrel lengths. But use 7.62×39 if you want to, it’s not useless and works great for hunting; as you say, “if it ain’t broke, don’t fix it”.

            Who told you 5.56 and the M16 were “first made for full auto spray and pray”? That was never the point. Please, read The Black Rifle before commenting on these things.

            Yes, you would teach flicking the selector to full auto in one motion in the event of an ambush! That is what automatic fire is most useful for!

            The selector on an AK is as you say, but they’re totally different designs. The AR-15 uses fine motor skills, and it’s theoretically difficult to flick the switch all the way to the automatic position. The AR-15 was, yes, more of a machine carbine than the American rifles that came before, but remember that those rifles were virtually literally designed for static range “gravelbelly” type shooting.

          • iksnilol

            Won’t try to discredit SCHV, quite the opposite. 5.56 is not fast enough. If you want high velocity go faster than 1000 m/s, Maybe go for something like the Swiss 15.2 mm (just scale it down of course). + you would get something that is more FA resistant due to no rifling to wear out.

            7.62×39 is way more versatile than 5.56, if you have used subsonic loads you know what I mean. If not, then you most likely don’t know what I mean. I mean, if it wasn’t that good then why is 300 BLK getting so popular? It is essentially a western compatible clone of the 7.62×39.

            The M16 was made with full auto in mind, the straight line design, the easily controlable cartridge. You can’t deny it comes from the era when they wanted as much lead in the air as possible (remember the Duplex loads?). The era when they thought that shooting many rounds could compensate for bad marksmanship. Thankfully they abandoned that train of thought. The AR-15 was adopted at the conclusion of Project SALVO.

            Also, why would you want to run out of ammo in quite literally 2-3 seconds in an ambush? You can rapid fire semi auto while not running out as fast. Remember, 20 round mags and early on soldiers didn’t even carry that many spares (though that was when it was new, don’t know about later on). Going full auto in an ambush is pretty much one of the fastest ways to get yourself killed.

          • Since drag goes up nonlinearly with velocity, and flechettes are as of yet not proven to be feasible, it is my opinion based on my ballistic studies that about 2,800-3,200 ft/s velocity is optimal for an infantry rifle cartridge. Going faster than 1,000 m/s (3,280 ft/s) is wasteful of powder, as the bullet quickly comes down from that velocity.

            I have fired subsonic ammunition, but in the US suppressors are heavily regulated. That’s changing as inflation carries on, but it’s still true. So maybe there’s additional versatility for Europeans, I admit.

            I think you totally misunderstand the point of that line of development. Again, I’d urge you to read The Black Rifle before commenting on this, as your understanding of the matter simply is deficient. I could burn a lot of time correcting it.

            Go ahead and forward your last paragraph to 4-man special operations recce teams, as going cyclic until the enemy leaves you alone is literally their doctrine.

          • CommonSense23

            4 man special operations recce teams have learned the hard way cyclic is not the way to go with a rifle. Those are old SOPs that went out of style real quick in the last decade and a half. It’s old Vietnam tactics from when a heavy load was 80 pounds which allowed a team to break contact real fast, not a 180 pounds.

          • Joshua

            The other issue is that while cyclic is fun controlled, accurate semi-auto fire can be just as suppressing as full auto fire, so long as your rounds are accurate.

          • CommonSense23

            I feel most people while understanding the concept of suppressive fire, they don’t actually know how to implement. I don’t know how many times I have been told to shoot my suppressed rifle just in the general area of a contact, even if I can’t see the contact. By people who should know better. I mean a guy who initiates a ambush with a belt fed, from a position of cover, with no ear pro, isn’t going to be scared by noise, but impacts within feet of his face.

          • For better or worse, that is how infantry combat is done most often. Evidence of this are the great number of combat videos available on YouTube.

          • That’s news to me, but then I get most of my info on the subject from WeaponsMan, who’s been out of the service for a while at this point. I am definitely not as much of a tactics guy as I am a hardware guy, nor do I think I should try to be.

            Out of curiosity, why did using full auto for breaking ambushes/contact fall out of favor?

          • CommonSense23

            A lot of reasons. One it came from Vietnam. Where do to a lot of factors, it was a somewhat appropriate tactic. The fact that those sorts of contacts happened at real close range, typically 30 meters or less, in incredible amounts of concealment. So at those ranges you might not be able to see the guy, but you have a vary good idea of where you are taking fire from. Ammo conservation isn’t the same problem for a SR team working deep in North Vietnam as one doing it in Afghanistan or Iraq. Those guys in Vietnam were often running from N.V.A. battalions. Doesn’t matter how much ammo they save, they won’t be able to hold the fort for long, they got to run. There have been a couple 4 man or less SRs in Afghanistan that have been able to wipe out the attacking force(the largest one was roughly 200 men) over hours without CAS. And probably the most important, is we are just much better gunfighters than the guys in the past. I am can keep 7 rounds a second from my SBR into a 4inch circle at 7 yards. The guys in the past just weren’t trained for that type of shooting. Couple that with optics. Its why automatic fire from a well trained shooter using a rifle is pretty much dead these days.

          • Uniform223

            The term “spray and pray” is just that. Unload a full magazine as fast as you can in the direction of hostile contact to create the first moments of covering fire so your dismounted element can get to cover and concealment. It is true that in the first moments of hostile contact the one that produces the most amount of “fire power” in the first moment often prevails. Though its not just dumping mag after mag going through belt after belt. One has to be mindful and smart about it. You have your suppressing element and you have your maneuvering element.

            Unfortunately during the early adoption of the M16 “spray and pray” wasn’t used the same way how I was trained use it now. You were right on a few parts that US service members would use it in a moment of panic and only panic. It didn’t help the fact that US service members would incorrectly “modify” their rifles in an attempt to reduce its cyclic (mess with the buffer assembly and spring as well as the spring inside the magazines) thus resulting in premature malfunctions.

            To try and say that in a moment of panic the user of an M16 would instantly go to full auto is a bovine fecal matter. The selector goes as follows; Safe, Semi, and Auto (burst if its later M16s and early M4s). In my experience you have to make bit of an effort to by pass semi and go to the last setting.

          • iksnilol

            For me the whole spray and pray thing is panicy (is that a word?). You’re going to find more corpses clutching a rifle set to full auto than one set to semi-auto. If you look at footage from war zones you will see what I mean (not for the faint of heart).

            I am not saying that they went instantly to full auto, I was saying that a manual recommended wearing down the switch and making going to full auto instinctive. Which to me seemed like a bad idea, but then again I am not a millitary nor a strategist so what do I know?

    • Cookoff would be a function of chamber temperature, not the gas system. One thing that’s interesting to note is that the M4A1 (Gov’t profile) also performed better than the M16A2, so I wonder if the very short Galil barrel was helping.

      Otherwise, I don’t know why a Galil would have a cooler chamber than an M16A2.

      • RealitiCzech

        Lower rate of fire?

      • CommonSense23

        A shorter barrel should by all accounts cool quicker than a longer barrel. If you have a hot rifle, you lock back the bolt to help it cool. Same principles are working at the muzzle. The question is how much of effort it will happen

    • RealitiCzech

      BTW, why is the Galil smoking under the handguards, while the AR isn’t? My piston AR does this as well. I know piston designs get hotter in that area than ARs, but I’m not sure what’s burning off. Excess lube? Paint?

    • Don Ward

      Israel has rednecks too?

  • It was Faxon Firearms’ pleasure to have been part of IV8888’s
    test. It was great to see the barrel pushed to its maximum and we are very
    proud of the barrel and its results. It performed well within and above

    Before we get started, we want to praise IV8888 for conducting
    the test. It helped dispel quite a few myths attached to the AR platform.
    Namely, that they can work with steel-cased bi-metal bullets, and that they can
    be pushed a long ways under extreme duress.

    The primary cause of the failure was high temperatures causing
    the barrel’s stainless steel to anneal (soften) which could not contain the
    pressures. The barrel did not literally “melt” but we understand it as a figure
    of speech.

    For those interested in the details want to understand the
    performance of the barrel and weapons platform, it’s important to understand a
    few points:

    – IV8888 used one of our barrels without consulting Faxon
    Firearms. We only mention this, as the chosen material was not ideal for the
    test. IV8888 used a 416-R stainless steel barrel instead of mil-spec 4150 CMV,
    which Faxon also offers and would have recommended to simulate more real-world
    usage. We have offered to them a 4150 barrel of any profile if they want to
    re-do the test.

    – 416-R stainless steel is not as thermally conductive as 4150
    CMV, meaning it does not exhaust heat as fast as regular “mil-spec” barrels. At
    these extreme temperatures (looking at the material, we estimate the barrel was
    over 1200 degrees prior to failure), the 416-R cannot transfer heat as quickly
    as 4150, causing an earlier failure. 4150 would also have failed, but a bit
    longer (we estimate no more than another hundred rounds).

    – IV8888 used a “Gov’t” profile barrel, meaning there is a taper
    to a relatively thin point behind the gas block. It was expected that the
    barrel would blow at the spot shown. It is the thinnest portion. A heavier
    profile such as the M4A1 would have lasted significantly longer. This is the
    same reason machine gun barrels are typically inches thick.

    – The duty cycle used during the test far exceeded any real-world
    or military scenario. Under real-world scenarios (even multiple full-auto
    magazine dumps in short order), the barrel would have continued to perform admirably.
    Under this extreme scenario, the temperatures inside the barrel near the
    chamber would be expected to physically degrade the rifling.

    – The gas tube is designed to fail on an AR, but was not expected
    to under this test. Gas tubes are designed to rupture under extreme round
    counts with C-mags and belt-fed weapons. See MAC and BigShooterist which tested
    a DI belt-fed weapon. It fails after about 125 rounds straight. The gas tube in
    this test would have ruptured first if a) reload times were faster or b) larger
    magazines were used.

    o This should help put shooters at ease who use normal magazines.
    A gas tube simply cannot get hot enough quick enough using 30 round magazines
    (as originally intended).

    o It also centers on physics. It is noted that the tube constantly
    changes temperature, a few times almost rupturing. A gas tube as significantly
    more surface area to volume than a barrel and thus can disperse heat faster.

    o It also shows why almost all machine guns are piston-driven.
    Pistons, with their higher density & larger volume of material can handle
    extreme heat better than DI under full-automatic conditions.

    On a safety note, we never recommend anyone do this, ever. When
    a barrel or gas tube ruptures, very high PSI gas is exhausted at velocities
    significant enough to cause physical harm to a shooter. We conduct our testing
    on benches using blast shields and armor.

    To answer a few other comments we have seen:

    1. A CHF barrel would not have lasted longer than a normal 4150 “mil-spec” barrel
    assuming the same profile. At these extreme temperatures, the material used is
    the highest indicator of longevity.

    2. A thicker barrel would have lasted significantly longer in terms of rupture, but
    the rifling would have degraded almost equally across any barrel profile. This
    is why machine gun barrels are changed often. Its to conserve the rifling, not

    3. A carbine gas tube would fail sooner. It is exposed to higher temperatures and pressures during firing. Likewise, a rifle gas tube will fail slower.

    We are very happy to answer any questions on the test and assist
    from a manufacturer’s perspective.

    Thanks for watching and reading!

  • This happened to me when I was still in the service. My issued Colt’s gas tube blew and I had a straight pull bolt action after that. That happened after a lot of Full Auto fire. You know what I took away from that?
    Don’t run your rifle like it was a Machine Gun. Because it’s not a Machine Gun… it’s a bloody rifle. Treating your rifle like a Machine Gun only proves that Machine Guns are better for use as Machine Guns. You run a rifle that’s designed to be light and accurate like a Machine Gun – of course you are bloody well going to have some problems.

    • sianmink

      If you’re doing mag dumps with your M4 you are either covering an assault or covering a retreat.
      If you have to do more than 3 of these in a row, then you’re probably doing it wrong. Like you said, that’s supposed to be the 249’s job.
      What happened to force you to blow your gas tube? How many mag dumps?

  • sianmink

    we’ve got 2 sets of melted AR handguards from some somewhat excessive torture testing involving full-auto lower. I’m talking 360-400 rounds as fast as you can feed them. the gas tube was incandescing quite happily. Luckily we didn’t start any fires, but damned if the thing just didn’t keep running.

  • Rob

    That’s an informative demonstration, but I am amazed that the man didn’t have a face shield, vest and welding gloves at a minimum. Yikes.

  • Lance

    I agree and I will say if any one is mag dumping that fast they are not treating there weapon properly or using should training for a gun fight. For all th hype piston guns have. Strange the elite of the Military passed over SCAR and others for the M-4A1 and its SOPMOD 1 and 2 kits. If you treat your wepon right then it work and last forever. Think training was the biggest lesson learned from WANUT.

  • Jack

    It would be fascinated if they conducted the same test with an AK to see how many rounds it can go through before failure. I would have thought that the gas tube would have failed first before the barrel. However the gas tube does cool much faster than the barrel.

    • Some additional data points:

      Note that neither of these tests have the kind of round counts that the Colt or IV8888 tests do, but the one with the AK and drum magazines is pretty brutal indeed. That gun barrel’s temper is surely totally gone; I would retire the rifle after that test, for certain.

      The test of the Vz. 58 is a little disappointing; there’s an edit where it looks like they had a malf, but you can’t be sure. They do put a lot of rounds through it, but not exactly in one sitting.

  • MPWS

    That experiment by Iraqvet got little risky but luckily….. he made it safely. I have seen test in which more shots were pulled, but it was MG firing from open bolt. The barrel was so hot that you could see bullets (visible thru barrel wall) taking off. it was also quite a bit thicker.

    This surely is testimony to durability of M16/ AR15 system and is learning and worthwhile even to older vintage such as myself. Thank you! Shoot safely!

  • Grindstone50k

    I should’ve popped some popcorn before scrolling down to the comments.

  • Georgiaboy61

    Crew-served weapons – squad, company and battalion-level general-purpose and heavy machine-guns – are better-designed for sustained fire missions and fire-support. The M16/AR15 family was never really intended for that role – and it is no surprise that the carbine’s barrel and/or gas system failed during prolonged use.

    Ironically, our forbearers as long-ago as the First World War understood this perhaps better than we do today, given the sustained rates of fire expected of MGs during trench warfare. A water-cooled Maxim or Vickers could function reliably for hours at a time under sustained cyclic use – if it was kept cool with sufficient water in its cooling jacket. The horrific death tolls in the battles of that war certainly prove the point (although artillery was as-lethal a kill as automatic weapons fire, at least on the western front).

  • It’s possible there are differences for different variants, barrel lengths, etc, but the carbon that is ejected into the receiver is – by a great margin – “soft” carbon that can be wiped off with a finger.

  • Have you considered that the reporters you are citing may not be playing fair, either?

    The M4 Carbine did very well in the IC competition, so journalists have naturally taken to making a squawk about the Army killing the program because it didn’t want the M4 to face its competition, which is sort of getting the story backwards, isn’t it?

  • Joshua

    It was fair. Everyone knew M855A1 was the new round and everyone was given 10,000 rounds of M855A1 to fine tune their rifles before the competition.

    The M4A1 has received no changes since M855A1 came online, this was actually a major benefit to the competitors.

    It’s not the Armies fault no rifle managed the required 3,000 MRBS.

  • Engeewannabe

    At risk to be over cunning I guest direct impingement influences heat dissipation. Gas tube glow white hot after 3rd 50 mag (video 1). Break point is under gas tube and barrel ostensibly “blend” there as well (video 2). So barrel re-heats twice near gas tube, each round. It is only a opinion about “how works” it is not a direct impingement criticism.

  • ozzallos .

    Torching barrels is great and all, but you also need to observe regular usage to get an accurate picture of the failure point. I mean, yay that DI performs well under extreme conditions, but extreme conditions aren’t the same as regular wear.

  • ozzallos .

    I know magpul furniture is pretty and all, but I never could warm up to the idea of potentially flammable, molten plastic around my barrel for exactly this reason.