Heat is bad. DI rifles get hot. DI rifles are bad.

AR/M4 AR General Discussion
1

…the title is meant to be in jest, by the way. http://vuurwapenblog.com/2010/04/04/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. I’ve never found touching the bolt carrier after firing to be a necessary exercise, but some find it to be very important, or at least fun. However, it’s not as if the 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 shocked – honestly – 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.

2

You and your precious science! Don’t you realize the way to win teh internetz is by shouting your armchair-based opinions as loudly as possible with no actual facts to back them up?
[/sarcasm]

Seriously though, once again you’ve taken the time to gather very useful comparative data that sheds light on the realities of modern weaponcraft; Kudos!

I was especially pleased that you gathered data on bolt temperatures, since piston pushers are forever crowing about how much cooler their bolts are without furnishing actual data…:rolleyes:

Keep up the good work, I check your blog regularly and always enjoy your insight and honesty.

3

Yes, Thank You for sharing the information.

Until I discovered this site, I was pretty well set on purchasing a Piston AR. The more information I come across regarding the standard DI AR series of rifles it just solidifies my amateur opinion that for many it may just be the best choice of all the ARs currently available, especially considering the availability of standard (non-proprietary) parts.

SkiDevil

4

good info. you wouldnt happen to have a pencil barrel to test too.

5

yea the bolt temperature myth debunk is also confirmed by another article that was posted a couple weeks ago. They fired a buncha rounds and the guy touched the bolt on purpose without getting burns.

My only suggestion would be to take more data points in that 10 minutes rather than just the four. Maybe every minute instead of these intervals

Thanks for the test.

6

87, any chance you can take actual chamber temperature measurements?

Im wondering if piston guns tend to have cooler chambers although i highly doubt this is the case since barrels should heat up uniformly whether they’re DI or Piston operated.

I find it hilarious that people will bitch about the temperature of the bolt but they will never bitch about the temperature of the piston operating parts. There are more moving parts and springs in a piston gun than there are in a DI gun that are exposed to heat.

7

I have the feeling chamber temps will be very similar if not the same

8

Any chance you would have a linky to that discussion?

9

This myth has always pissed me off. Often perpetuated by liars like LWRC and many other bottom feeding piston peddlers.

Some dude over on Silencertalk made a thermal video of the DI with a silencer mounted. The point was to see how a can heated up. But it was quite interesting and undeniable to see the LACK of heat build up in the bolt group area.

Every gun will eventually get a hot barrel and chamber, but the idiotic myth that the DI is significantly worse is an absurd pile of crap.

10

Not going to disagree.

I posed the question not to support what i believe to be largely a myth but to have someone provide some scientific evidence to dispel it.

11

Mark M is right. LWRC is below bottom feeders. Worse customer service ever! They don’t care if you are on a LEO/MIL account. They treat you like crap and they have supervisors that cuss a lot. I counted the LEO supervisor saying Gd Dmn 7 times in one conversation. They are not professional. I will NEVER spend another penny with them. They do not stand behind their products. The piston driven crap is just crap. DI has been working and will still be working. There is a reason why the Big Green Killing Machine has NOT gone to Piston ARs. Now after a couple of thousand in the hole I know why. Do not learn this expensive lesson like I did. LWRC sucks balls.:mad:

12

I guess my only question is, at what point, or after how many rounds fired does the
" DI’s are more dirty than Piston systems" come into play if at all? That seems to be
the other major part of the DI vs Piston comparison.

13

Unsuppressed DI weapons are without a doubt “dirtier” than piston guns.

The problem lies in the fact that most internet commandos (im one of them) dont do enough high volume shooting to realize just how far you can push the DI system, DIRT AND ALL, with just a little lube.

14

I think you just discribed my volume of shooting to a T. But for some reason I find myself still wanting a good piston setup. Just thought of another, what about site recovery? I’ve also read piston guns have less recoil and thus have faster site recovery. Ok, I think I’m done. Thanks for the reply to my question. Didn’t mean to step on the thread.

15

Here’s a link to an article talking about exactly that. It’s all about proper maintenance and understanding how the system is supposed to operate.

http://www.defensereview.com/m4m4a1-carbine-reliability-issues-why-they-occur-and-why-theyre-our-fault/

16

Thanks again for the reply, and the link.

17

what about site recovery? I’ve also read piston guns have less recoil and thus have faster site recovery.

I’ve read exactly the opposite.

18

Who told you that BS? Its the that piston will have a sharper recoil impulse. Maybe you heard it wrong.

19

Every piston gun ive ever shot has had a sharper recoil impulse. Beyond being sharp its quite different in the way the recoil feels when compared to an average DI AR.

What i dont understand is what exactly are piston guns supposed to be fixing that is so “broken” with the AR system?

The majority of malfunctions that we see have nothing to do with the actual gun (if its set up properly) and more to do with the magazines and the ammo itself.

I hate to turn every thread into a piston versus DI debate. Im really not a fan of pistons but i have no personal bias against them except when people love to flog them as being the solution to the AR “problem”.

20

Thanks to 87GN for the cool study and great info.

I have an Adams Arms Kit on my 10.5" SBR. I have been pleased with it, no malfunctions in over 1400 rounds. I’ve shot enough rounds through it at one time to get the barrel smoking pretty good. It has always functioned.

I don’t have any buffer tube wear from carrier tilt (yet). I did have some wear on the upper from the cam pin. I noticed this at about 1000 rounds and put in the POF roller cam pin. 400 rounds later the wear hasn’t gotten any worse. However, because there was wear already there, and because I have shot few rounds (comparatively), I can’t really say ,yet, how effective it is.

I do notice less recoil with the AA kit compared to when it was DI. Although most AA users say they have less felt recoil, I can’t say if that is really due to the piston system. First, I’m using a heavier buffer with the AA kit. Although AA says their gas block is only 3 oz. heavier than a standard FSB, it feels heavier to me, and the rifle feels more muzzle heavy to me than when it was DI.

The AA kit is much cleaner (on the BCG). No comparison there, in my experience. I have a 12.5" SBR that has about the same number of rounds through it as my AA Kit SBR and the AA kit SBR barely had any carbon build up on the BCG after 1000 rounds. BUT, there is carbon build up where the op-rod vents near the gas block. Had some build up on my folding front sight. I want to note that my 12.5" SBR DI rifle has been just as reliable as my 10.5" SBR w/ AA kit.