Do gas pistons keep the barrel and chamber cooler than direct impingement?

[Bartholomew Roberts]

I just got back from the range and can categorically tell you that the chamber and lower stay way cooler and cleaner.

How can you can categorically tell us that the chamber stays cooler? Did you measure the temperature? How did you arrive at that belief?

The barrel, seams to be cooler and overall the upper cools a lot faster.

What did you observe that made you think the barrel might be cooler? Stating "The barrel was cooler" doesn't provide any useful information to this discussion at all; because for all I know, you reached that conclusion by noticing that the barrel was black. Telling me how you reached that conclusion is a lot more useful if you are trying to actually share information with the rest of us.

Realistically, the barrel might actually cool faster. After all, the gas piston is connected to the barrel and it does have a lot more mass than the gas tube. So it would be better able to absorb heat from the barrel. However, the SAR tests don't seem to show any major cooling effect on the barrel or chamber.

The fact remains the GP rifle runs alot cooler in the entire rifle including the barrel and chamber over a DI rifle.

If it is a fact, then all I am asking is for someone to show me that information.

I already posted pics on the temps, Im not going to continue on that subject until I take more pics and show more temps with a few examples in comparison.

Frankly, if I were you, I wouldn't continue on that subject at all until I had read some high school physics and developed a fairly sound methodlogy for your test. Your previous posts not only show that you don't understand some basic elements concerning physics, they also show that you don't know when to quit even when you are blatantly wrong.

On the one hand, I am really interested in seeing someone run the tests you want to do. On the other hand, why am I going to trust the results from someone who was arguing that a gas piston had caused his bolt to cool below ambient temperature?

Already others are seeing the same differences in temps I mentioned, aside from multiple manufacturers their testing, and aside from multiple independant tests.

Great! I'll take all the data I can get. Please list for me all the manufacturer testing showing that the barrel and chamber of a gas piston rifle run cooler than a direct impingement rifle that you are aware of. I would also welcome any independent tests showing the same. Finally, I would welcome any anecdotal stories concerning that issue if they also include factual observations that led to the conclusion and/or real data on the subject.

So far, I've been at this for over a week and despite "multiple manufacturer testing", "multiple independant tests" and a few anecdotal posts, I have exactly one source of hard data - the SAR review of the POF piston. That review shows a whopping 10F difference at the chamber after 264 rounds - and it doesn't account for the difference in barrel profiles (16" M4 vs. 16" HBAR Fluted Piston), problems with IR thermometers for temperature measurement, or any of the other variables that might explain a 10F difference.

I would also argue that 10F is a long way from a LOT cooler and ask how that is any practical benefit (especially since you need to fire 3 Beta-C mags with 90 second intervals to see that difference)?

 

[Owen]

PMROY

How are you measuring the chamber temperature?

 

[MCMXI]

In an earlier test published in The Small Arms Review in September of 2006, the authors had this to say ...

"The chamber area temperatures in the P-416 ran surprisingly low, between 80° - 92°F after 20-round and 30-round bursts with
little time for cooling in between. The standard M16 gas system averaged between 86°F and 120°F under the same conditions."

So it's very possible (and probably likely) that for most people under most situations, the temperature differences between GP and DGI for the bolt, carrier and chamber are insignificant. It's a pity that the DGI AR failed (gas tube) after only 264 rounds in the second test published in The Small Arms Review in March of 2008. I wish they had installed a new gas tube and continued just to show how much carbon fouling occurs. One thing does stand out from the test though, the POF bolt face reached a temperature of 120°F after 1036 rounds whereas the DGI bolt face reached 115°F after only 264 rounds. It's VERY likely that the DGI bolt face would have reached a much higher temperature after 1000 rounds due to hot gasses and increased friction from carbon build up. Anyone that has cleaned a DGI AR can attest to the amount of carbon fouling after only 50 rounds, not to mention 20 times that number.

Regardless of the test above, as I mentioned, the temperature differences are most likely insignificant for most of us since we're not going to waste 1000 rounds in 20 minutes. However, the fouling issue is enough justification (to me) to choose a GP AR over a DGI variant. It would be interesting to test GP, DGI and open bolt ARs to compare bolt face, bolt carrier, barrel and chamber temperatures.

 

[Bartholomew Roberts]

Thanks for pointing out the earlier article, 1858. I noticed a few things in that article:

1) In some recreational full-auto shooting, the DI gun hit 126F; but according to the chart in the second article the DI chamber temp didn't get that high even after 264 rounds fired in 100rd full-auto bursts.

Was there a major difference in ambient temperature between the two tests or does this give us some idea of the margin of error in the testing method?

2) The 9.25" barrel POF has almost identical chamber temperatures to the 16" POF used in the second test, even though the 16" has a lot more mass to heat up. This could be explained by the harsher conditions of the second test; but then how did the 16" DI gun get hotter in the first test shooting under the same conditions as the 9.25" piston?

3) I noticed that POF doesn't claim the difference in chamber temperature is due to the gas piston. Instead they claim the cooler chamver temperature is a result of the proprietary barrel nut used in conjunction with the Predator rail to act as a big heat sink.

 

[PMROY]

Excuse me gentlemen for my unprofessional observations. I greatly enjoyed your discussions on the thermodynamic variances measured after completing a shooting test. My observations were simply a side by side comparison of two similar rifles, firing the same type and amount of ammunition, at the same ambient temperature and relative humidity. If we really want to get technical, the first law of thermodynamics states that, energy cannot be created only change from one type to another and thus a specific amount of heat will be the result. How and how quickly the heat is dissipated, will depend ultimately on the material of the barrel and in my humble opinion the total surface area were the heat is transferred to be cooled. My basic rationalization is that a gas piston rifle has a much greater surface area to dissipate the heat created by the primary source. The gas piston assembly versus one gas tube. Again it was just a very basic observation of my experimentation without using a control group and diving into a full blown statistical analysis of the results.

 

[SHvar]

The facts are there, regardless of what opinions you have. Do you have a GP rifle to compare to a DI rifle, or access to one to compare in person?
If not how can you say that whats being experienced first hand by those with access to them both is wrong? Have you experienced or have you witnessed in person otherwise?
Obviously you cannot, thank you.
I dont need to argue physics, or anything, I have first hand experience to stand behind what I said. I even posted pics, but thats your choice to believe what you want to, this is America, we have that freedom.
By the way, I already know that I can fire my GP rifle and handle most of it right away without gloves and not get burned, try the same with a DI rifle, I wont do it, I knew many years ago how hot they get.

 

[SHvar]

By the way.
"someone who was arguing that a gas piston had caused his bolt to cool below ambient temperature?"
Thats not what was posted, maybe you should go back and get educated in basic reading comprehension.
What was said was that the bolt and carrier were below air temperatures, it was in the 90s that day, the bolt was very cool after firing over 200 rds, below those 90f plus temps. The gas piston did not cause it to cool below that, it never got heated up in the first place by firing it. This is what I typed.
Im sure that if the rifle sat out for hours and hours in those temps beforehand, it would be equal to air temps afterwards. The point was that it never got hot in the first place from firing over 200 rds.

 

[Bartholomew Roberts]

My basic rationalization is that a gas piston rifle has a much greater surface area to dissipate the heat created by the primary source. The gas piston assembly versus one gas tube.

I understand the logic and it makes perfect sense to me that a gas system with more mass and surface area will take longer to heat up and will cool down faster. I just don't understand what you observed that made you feel the chamber of one rifle was "categorically cooler" than the other? Was smoke coming off one barrel but not the other? What did you see, hear, touch, taste, smell that made you think one was hotter than the other?

Thats not what was posted, maybe you should go back and get educated in basic reading comprehension.

A bold statement from someone who keeps quoting the temperature of the bolt and carrier of his gas piston rifle in response to a question about the barrel and chamber temperature.

What was said was that the bolt and carrier were below air temperatures, it was in the 90s that day, the bolt was very cool after firing over 200 rds, below those 90f plus temps. The gas piston did not cause it to cool below that, it never got heated up in the first place by firing it. This is what I typed.

What you typed is available for all to see on Page 2 and it seems you forgot to mention that your bolt and carrier started out at below ambient temperature when posting your temperature data. As a result, your temperature data is pretty much useless since I don't know if the starting point of the test was 80F and the bolt and carrier heated up 3F or if the bolt and carrier started at 20F and heated up 63F.

Of course, it isn't really relevant to this thread in any case since I never asked about the bolt and carrier to begin with; but it does illustrate why I would be skeptical about any data you posted. Not giving us the starting temperature is leaving out some important relevant information that could lead somebody to reach a bad conclusion.

By the way, I already know that I can fire my GP rifle and handle most of it right away without gloves and not get burned,try the same with a DI rifle

That strikes me as a test that proves nothing. For example, let's leave your gas piston rifle lying on the range in the direct sunlight of the Texas sun while we walk down to pull targets at the 600yd berm. Then when we come back, we can take my DI rifle that has been lying in the shade, fire 200 rounds through it and see which is easier to hold? I know from experience that the rifle that hasn't fired a shot will be much more unpleasant than the rifle that fired 200 rounds. Must be the gas system eh?

However, I've fired as many as 800 rounds in a day through a direct impingement AR and never had any trouble holding most of the rifle. The barrel was something I wouldn't grab; but then again, I doubt you are going to have much luck firing 200 rounds as fast as you can through a gas piston and grabbing the barrel either.

 

[SHvar]

From the POF article, although they did not collect data on barrel temps because the DI rifle failed at only 264 rds, where as the GP rifle fired straight through 1036 rds before they stopped.
The GP rifle reached a similar chamber temperature after firing 2.27 times as much ammunition as the DI rifle. Also after firing 3.924 times as much ammuntion as the DI rifle the chamber temps were less than 9 degrees higher.
This stands to reason that if temps were measured at the chamber at 264 rds with the GP rifle they would be much lower, also the bolt face is close enough to the chamber to be effected by the same temps.
From the article.
"was 122° and that was recorded
immediately after firing 1036 rounds. The
maximum temperature the bolt face
reached in the direct gas impingement
rifle was 133° but that was only after 264
rounds" (almost 4 times as many rds fired and it still read 11 degrees cooler).
The temps Ive experienced with my GP and DI rifle at the same air temps, carried in the same bag, to the same range, and stored in the same cabinet in the same room beforehand show that the GP rifle stays noticeably cooler to the touch, and cools down faster than the DI rifle.
My DI rifle has a surefire quadrail which is definitely far better to allow cooling of the barrel, the barrel on the DI rifle is a full 1 inch heavy barrel which allows it to cool faster than a standard A2 heavy barrel, a Hbar barrel, or an M4 barrel like my GP rifle has. The GP rifle uses double heat shields inside of full plastic standard handguards which does not allow for cooling to occur so quickly.
Im sure the difference would be much more dramatic if both were equipped the same (no heat shields, alluminum quadrails made by the same manufacturer, same thickness and profile of barrel).

 

[Limeyfellow]

A gas piston rifle ejects only a small amount of gas to cycle the action. The rest is of super hot expanding gas goes the same way as any other rifle. Any differences are going to be too minuscule to notice outside of expensive temperature measuring equipment.

 

[Bartholomew Roberts]

From the POF article, although they did not collect data on barrel temps because the DI rifle failed at only 264 rds, where as the GP rifle fired straight through 1036 rds before they stopped.

To be precise, the gas tube (an $8.00 part on the DI rifle) ruptured on the DI rifle. SAR didn't mention what AR brand they used; but as the M4 Fire To Destruction test demonstrated, an M4 with a quality gas tube is capable of firing at a rate twice that of the one used in the SAR test.

However many commercial ARs are manufactured with the philosophy that it is better to have an $8-15 part fail (gas tube) rather than allow the rifle to continue to fire until the $200-250 part (barrel) ruptures.

This stands to reason that if temps were measured at the chamber at 264 rds with the GP rifle they would be much lower

Except we don't need to reason or guess. The SAR article included a chart showing the temperature differences at each stage of firing and they clearly show that at 300 rounds, there isn't 10F worth of difference between the gas piston and DI rifle in chamber temperature.

Not only that; but in an earlier SAR article on the 9.25" POF that 1858 linked to, the manufacturer states that the lower temperature is a result of their proprietary barrel nut - it has nothing to do with the gas system according to the manufacturer.

The temps Ive experienced with my GP and DI rifle at the same air temps, carried in the same bag, to the same range, and stored in the same cabinet in the same room beforehand show that the GP rifle stays noticeably cooler to the touch, and cools down faster than the DI rifle.

OK, but is that because of the gas system? What other differences are there between the rifles? As I understand it, the Bushmaster gas piston is actually the same as the POF gas piston. Does it have the same proprietary barrel nut?

 

[Joe Demko]

How does a proprietary barrel nut keep anything cooler? Two identical cartridges will produce identical amounts of energy. All that can be played with here is where that heat energy goes. Some will be retained in the cartridge case. Some tiny amount is in the bullet. What's left must go into parts of the rifle and into the air as firing gases leave the gun. I'm open to explanations on this, but it sounds hokey.

 

[waterhouse]

How does a proprietary barrel nut keep anything cooler?

I haven't seen the barrel nut in question (which POF claims is a "Patented, oversized barrel nut" and elsewhere as a "3.375" long oversized heat sink barrel nut"), but variables such as surface area and material will both have an effect on how fast something can dissipate heat. If you apply the same amount of heat to a solid ring and a ring of the same material but cut in a pattern similar to this:

the one with all of the extra surface area will get rid of the heat faster. As you stated, some of the heat energy goes into the rifle parts, one of which is the barrel nut. If, through design, materials, or both, that barrel nut gets rid of heat faster than another barrel nut, the rifle as a whole will be less hot.

I'm not claiming the POF proprietary ring does what they claim, merely that materials and construction both have an effect on how fast something cools off.

ETA: A little more searching and I found this picture of the POF barrel nut:

compared to a regular AR-15 barrel nut:

 

[Joe Demko]

In the beforetimes, they used to try to take advantage of that idea. That's why a fairish number of long long ago machineguns and submachineguns had barrels with cooling fins machined into them. The before people gave it up as needlessly time consuming and expensive for no real benefit. I admit I've never checked on any of my rifles to see, but does the barrel nut retain enough heat normally for something like that to make a difference?

 

[waterhouse]

Like you, I've never fired off a string and checked my barrel nut temperature, but the theory is sound.

I'm also not sure about whether there is any real benefit, but I'd bet with the right instruments you could see a measurable temperature change. The question is how much benefit do you get? If it cools the part from 300 degrees to 285 that probably isn't enough to make any real difference (note that those numbers are purely fictional and based on no data) but it is certainly measurable.

In the super competitive AR world, being able to say that your proprietary part "runs cooler" may get you a few extra sales. (once again, the part may work very well and lower temperatures dramatically, and I'm not trying to imply that it does or does not do what POF says.)

 

[Joe Demko]

Well, I can offer you this much: I have a DI carbine here with a one-piece aluminum free-float tube. It should act as an even better heat sink than that barrel nut. Through some pretty long firing sessions, I've never known it to get better than warm. This was at times when the barrel and front sight base would have been really ouchie to grab.
So, I guess that barrel nut probably does absorb and radiate some of the heat involved; I'm just not convinced it is a significant amount.

 

[hillbillydelux]

Um Not to ruffle any feathers but there really should not be a huge problem in figuring this one out. DI diresct hot gas from the barrel into the bolt. Therefore DI guns will have hot bolts. GP guns vent the gas out at the piston. So the bolt does not get as hot. However the difference in semi auto is irrelivant. Full auto is where the GP design shines. Thats it. No crazy scientific explination needed just a little common sense. Also neither design has any impact whatsoever on chamber or barrel temps.

 

[MCMXI]

As I understand it, the Bushmaster gas piston is actually the same as the POF gas piston. Does it have the same proprietary barrel nut?

For the record folks, my 2nd generation POF upper DOESN'T have that fancy POF heat sink barrel nut ... so maybe POF realized that it wasn't worth it or had little benefit to the weekend warrior. The Bushmaster GP upper comprises of POF's 1st generation system. POF has made some changes/improvements to the original design.

 

[PMROY]

Sorry, i was not explicit enough, I own a Bushmaster with the Gas Piston and a Colt LE, they both have the same amount of rounds down the barrel. And my observations were at the range, with my rifles, with the same type of ammo, etc. Gentlemen, are we getting to the point on this discussion, that we are going to need to hire a slew of scientists, a sterile testing environment, with the required measuring devices and before that, get someone to manufacture two rifles, to exact tolerances, one a gas piston the other DI? Can we not reach a concurrence based on all the useful and well researched data provided. After reading all of your posts, I understand the rationalization behind the different opinions and respect them all.

 

[Maverick223]

need to hire a slew of scientists, a sterile testing environment, with the required measuring devices and before that, get someone to manufacture two rifles, to exact tolerances, one a gas piston the other DI?

Do you mind?

 

[Bartholomew Roberts]

And my observations were at the range, with my rifles, with the same type of ammo, etc. Gentlemen, are we getting to the point on this discussion, that we are going to need to hire a slew of scientists, a sterile testing environment, with the required measuring devices and before that, get someone to manufacture two rifles, to exact tolerances, one a gas piston the other DI?

I was just trying to understand why you felt the one rifle was categorically cooler than the other. Personally, I couldn't give you a good idea of what the barrel or chamber temperature of a rifle was short of touching it - and if it was cool enough for me to touch, then I would probably be even less able to appreciate the difference.

If you just eyeballed it and decided that one looked cooler than the other, then that's fine. I wouldn't really describe that as "categorically cooler" myself; but telling me you eyeballed it and that one looked cooler at least gives me a basis to evaluate your conclusion. Stating that you observed that it is cooler without explaining HOW you observed that (touched it/licked it/measured it with thermocouples/measured it with IR thermometers, let my cat use it as a scratching post) doesn't really impart any information.

 

[PMROY]

I apologize again. I touched both of the barrels, chambers and after five minutes carefully removed the bolts. The difference in temperature was evident to the touch. As to what is evident to the touch?It varies from individual to individual, for some it could be only 10 degrees and for others 40 degrees or more before they can notice a variation. All I am saying, is that to My touch there was a noticiable, to me, difference.
I even waited ten minutes between firing of the rifles, to have enough time for my sense of touch to reach an acceptable baseline and not be influenced by heat transfer of the first trial. Nothing too scientific, just a little experiment, in an uncontrolled environment. The reason I did it was because I found the discussion interesting and wanted to see for myself. I wish we had the money to do a full blown out test of the question posted, I am sure we would get enough volunteers for the firing part( ammo provided, auto and semi-auto, no limitations, you just have to duplicate sending thousands of rounds down range)? Then again why don't we do it? I'll put my name on the list of volunteers.