Heat Dissipation: Two Schools of Thought

Heat is discussed fairly often on various internet forums, especially when two subjects come up: barrel profile and method of operation.

We often see comments about how light barrels heat up too quickly. This is partially true – a lighter barrel will generally heat up faster than a heavy barrel. The “too” part is where the problem lies. Too fast for a machine gun barrel? Most likely. When you’re putting out a sustained rate of fire that can reach several hundred rounds per minute, a light barrel is definitely unsatisfactory. However, if you have a rifle, and not a machine gun, a lighter profile barrel may not heat up “too quickly”.

Also, there are many comments about how cool piston/op-rod systems run. These comments seem to be applied liberally and generally; that is, you will often hear that all piston conversions “run way cooler” than standard DI weapons. However, it’s not as if the mere presence of the op-rod has a chilling effect on the barrel, which is a critical component of the rifle, to be sure.

So we have two schools of thought here: that a lightweight barrel profile is more appropriate for use on a carbine, and that the standard system of operation is not unnecessarily hot; and that a heavy (or fluted/heavy) barrel is more appropriate for use on a carbine (or maybe a carbine machine gun to be used for laying down suppressive fire), and that an op-rod allows the rifle to run cooler.

Recently, while doing some experiments with standard plastic handguards, I thought I’d also compare a civilian legal M4 clone, or close to it – a Spike’s Tactical M4 LE with a Knight’s Armament M4 RAS handguard – with two Patriot Ordnance Factory rifles. One is a P-415, which uses POF’s op-rod system, and the other is called the RDIK, and it uses a gas tube, just like a standard AR-15. However, it’s equipped with the same heavy fluted barrel, heat sink gas block, reinforced upper receiver, and single piece railed forend that the P-415 uses.

To complete this test, I fired 30 rounds of 5.56mm ammunition through each rifle, then measured the temperature of the handguards in four separate places, as well as the temperature of the gas block/barrel. I measured these temperatures immediately after firing, 1 minute after firing, 5 minutes after firing, and 10 minutes after firing, using an infrared thermometer. I also measured the temperature of the bolt face immediately after firing.

Here are the handguard temperatures:

Note that all three handguards got progressively hotter until sometime after the 5 minute mark. I found it interesting that the POF gas tube upper was within several degrees – up or down – of the P-415 op-rod upper during the whole exercise. I also found it interesting that every handguard had reached essentially the same temperature 10 minutes after firing.

Here are the gas block/barrel temperatures.

I was surprised to see how cool the POF RDIK gas block was after firing. There was a greater initial temperature difference between it and the P-415 gas block than there was between the P-415 and the Spike’s M4 LE. In addition, the M4 cooled faster than the P-415, as you can see, though each upper had roughly the same temperature loss profile after 1 minute.

This proves an often-overlooked point: while light barrels do heat up faster than heavy barrels, they also cool down faster than heavy barrels – apparently, faster than even a heavy fluted barrel. This also proves true a comment made to me by an industry professional while we were discussing this topic: that the gas block of a piston/op-rod rifle gets very, very hot.

As a side note, the temperature of the M4’s bolt immediately after firing was 94 degrees; the P-415’s bolt temperature was 88 degrees. The RDIK’s bolt was 89 degrees.

Although this was a very limited and rather unscientific test, it would seem that the vast majority of the POF rifles’ cooling ability comes from the heat sink barrel nut, handguard, fluting, etc, and not from the piston/op-rod system. I will do more extensive testing in the near future.



Filed under Tests

3 responses to “Heat Dissipation: Two Schools of Thought

  1. Joe

    On the issue of the bolt heating up:
    We have often been told, either by HK416 proponents or the SEAL on future weapons how hot the bolt of the DI M16 gets after firing a mag, and how it can’t safely be touched.

    This is pure theatrics. I’ve consistently put 6 magazines through an M4 in about a minute, and never had an issue touching or holding the bolt. The bolt never becomes that hot, even while the barrel and handguards get too hot to touch.

  2. Pingback: Heat Dissipation: Insulate or Circulate? Tube or Rod? « Vuurwapen Blog

  3. Alex

    Understanding that lighter barrels heat up faster and dissipate heat faster, why all the concern? I get the argument and science behind why people seek this information and have done hours of research on the subject my self. However: In all of my research I did not uncover one comparison of the M4 profile AR to any SIG 556. Though different guns when the barrels are in question the SIG 556 has a slim, lightweight profile barrel very comparable to the old Gov. Profile M16 Barrels, and smaller than the AP4 barrel profile.

    Old M16’s all had the featherweight profile as people like to say, and plastic handguards, and there are thousands of these guns still on the firing line. A friend of mine was issued a late 70’s vintage M16 by his local police agency. I am sure the National Guard (took real good care of these guns…) Most Guardsmen are not going to take care of their guns like a deployed soldier, cop, or avid AR lover. His gun is in excellent condition and is still dead on accurate out to about 100m.

    I have fire thousands of rounds through M4, M16, MK18, and Colt Commando, style AR platforms all having AP4 or Lightweight profile barrels. Yes they get screaming hot, they will blister your skin until it bleeds, Yes you have to let them cool down or you will ruin the barrel. This being said your average firefight in the civilian world is never going to last beyond a magazine, and if it does you do not need to be there, the SWAT team or the Marines do.

    Unless you have a class 2 machine shop, or a class 3 stamp for your gun for full auto fire you are not going to wear out the lightweight profile barrel. You cannot pull the trigger fast enough to create that kind of heat.

    Do not forget that the AP4 or M4 barrel with the M203 cutout is only slightly larger under the handguards than the featherweight barrel, and a real M4 with a 14.5 inch barrel not a 16… does not have that much more metal out past the gas block. M249 SAW barrels are proportionally thin considering the type and intended purpose of the gun. Several reports of these guns and even the M60’s with barrels so hot you could nearly see the bullets passing through the bore.

    Even in an “end of the world” situation unless you are part of a military unit the probability of you needing full auto is slim, and even so not something you want to do unless you have an endless supply of ammo. It takes thousands and thousands of rounds of practice before you learn how to control a full auto machine dumping a 30rmd mag into a 12×12 square, out past 25m a 12×12 square is a about a 3ftx3ft square. You will be more effective and “scary” if when the bad guy hears the report of your gun and then falls over dead, then him living and being intimidated by full auto fire, knowing your out of ammo, and getting up and shooting back making you fall over dead. Don’t forget with practice you can lay down accurate and lethal “suppressive fire” with a semi auto gun.

    But for the sake of the argument and question at hand, There is nothing to worry about in purchasing a lightweight or M4 barrel. Just don’t think that you can shoot a thousand rounds back to back. There is a reason why 249 gunners carry extra barrels.

    When considering accuracy, Heat is bad, the hotter the barrel the less ridged therefore less accurate. Heat also causes expansion causing both your chamber and bore to expand and become “larger” think about shooting a .270 through a 30-06. You can still hit the target at 100m but its going to be way off, but still on paper. Though an extreme example, an easy one to understand. Barrel length also is a factor in this relm, a longer hotter barrel is going to “sag” and a shorter one will not, that is just simple physics. With this in mind a heavy barrel may stay hot longer but it is going to give you better accuracy than a super hot featherweight barrel.

    Again, there a a million different arguments and variables to the equation that you can throw into the mix and the end result will always be different. The US military is not going to intentionally issue something that is going to fail. The bidder might be the lowest but the parts probably last the longest. Uncle Sam does not want to pay for the same thing twice.

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