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. He can be reached via email at [email protected]


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