Piston -vs- DGI systems test in SAR

AR/M4 AR Technical Discussion
1

Interesting article in the March SAR issue. They ran 1,000 rds through a piston upper and a std direct gas impingement upper. They did 100 rd Beta C mag dumps, full auto, reloading as fast as possible. The results were interesting. The piston upper ran like a striped ape and showed no signs of slowing down. The std upper puked after 264 rds. It had a “gas tube failure”, which I take to mean it melted down and sprang a leak somewhere, as it no longer had sufficient back pressure to cycle the bolt.

Temp readings were taken at several locations. The bolt face on the piston upper reached 122 deg F after 1,036 rds. The DGI upper reached a peak of 133 deg F after 264 rds. The chamber temp of the piston gun reached 131 deg F after 1,036 rds. The DGI gun reached 115 deg F after 264 rds. The gas block temps were interesting. The piston upper reached 498 deg F after 200 rds and peaked at 697 deg F after 1, 036 rds. The DGI upper hit 339 deg F after 200 rds.

So the piston upper was found to transfer heat at a much slower rate than a direct gas impingement system. Big duh there. But the interesting thing is the gas block heats up much hotter because the gas is stopping there. The gas block is taking one for the team by absorbing a tremendous amount of heat, as the piston and op rod continue, without the gas, to cycle the bolt. But even at twice the temp of the normal gas block, the piston upper ran like a champ.

So what does this all tell us? The most striking thing that I see out of it is: to run repeated full auto mag dumps like a SOF team doing an IA drill, a piston upper is very desirable. If this very heavy firing schedule is you idea of a good time, a piston upper might be a good idea.

With a std, direct gas impingement system, it would be wise to limit full auto mag dumps to 5-6 30-rd mags. Yeah I know they did it in “Heat”, and yeah it looked really awesome, but you will probably damage your weapon doing it.

What about semi-auto fire? I would be interested in seeing what kind of schedule would be equivalent to around 264 rds of full auto fire. I have cranked out over 1200 rds in a 8 hr period in a carbine course, and I never noticed my gas tube glow cherry red, as it did in this test. I would have to assume if a carbine can stick to this kind of schedule, then you will probably be OK in most any tactical situation.

The only time I ever stopped a carbine was was in training, doing a break contact drill, where I went through approx 200 rds of blanks, full auto, which carboned up the reciever so bad, the bolt stuck in the locking lugs. The gas tube was cherry red and the handguards melted down at the front corners.

So while I’m not a big fan of the piston uppers, as yet, I do see their uses for the extreme upper limit of the envelope. They may be the answer to SOF’s problems with the M-4.

2

Yes. This proves it. The millions of people with FA registered lowers, LLs or RDIASs AND millions of dollars to spend on ammo need to buy piston uppers…

3

I dunno about that, but if you are going in harms way and expect to need full auto fire, then a piston upper makes very good sense.

4

While probably a cool test to conduct, the failure of one upper at 264rds does not give any empirical data with which to hypothesize anything else.

It shows that one upper had a failure at 264rds. That’s all, and that’s it.

I had mentioned the gas block superheating on piston uppers to someone recently offline. Glad to see that was born out with testing. It means the gas port is also being subjected to superheating. :eek:

5

I think they are currently testing DI uppers under FA over in the middle east right now.

I hear they did this back in the late 60’s in a place called vietnam.

6

They may be the answer to SOF’s problems with the M-4.

Add suppressed fire and you will know why the 416 was born.

7

I have about had my fill of your smartass tonight. Its getting old, and getting old fast. Either discuss things in an adult manner, or find somewhere else to discuss.

8

I agree. It’s too small of a sampling to make any accurate judgements. That is interesting on the gas block and gas port though. That had completely slipped by me. I wonder how much the wear rate is increased?

9

The two biggest areas I see where a piston setup has an edge on a DI system, is heavy sustained full auto fire, and supressed fire.

10

One note to all this: the authors claimed they were not looking at barrel heat, per se, but a head - to- head test of the gas systems. It would appear that a piston upper does heat up at a much slower rate, as far as the gas system goes, but would the barrel be just as hot as a DGI system?

According to BR vol. II, the barrel must be brought to a temp of at least 1,100 deg F to lose structural integrity. Gov’t testing, in the wake of catastrophic barrel failures of M-4A1’s in the early 90’s, showed that for a barrel to lose structural integrity, where catastrophic failure would occur, between 540 and 596 rds had to be fired, full auto, in 3 to 3 1/2 minutes. This study resulted in the beefed up SOCOM barrel.

So there is something at work here. These guys got off about twice the rounds necessary to blow a std profile M-4 bbl, with a very similar firing schedule. So it would appear that there is a benefit to keeping the hot gas out of the chamber area.

Be interesting to see where this all goes. For the record, I’m not pimping piston uppers. I plan on keeping my DGI systems running for a long time. I just thought this test was interesting.

11

Ok, I’ll explain my joke and apologize if it seemed I was making fun of Diz.

These tests can be formulated to absolutely destroy one test subject but stay well within the operating abilities of the other. This is just such a test. The testers knew that a few 100 round dumps would cook the gas tube and make the gun appear to be a failure and they did. The pistol doesn’t fail the same way.

It makes it look like the DI guns can’t handle FA fire when in fact they can- that was my point. The DI guns have been able to handle realistic FA fire for a long time. They’ve had problems along the way but luckily for all of us fans, they’ve survived and are still going strong.

This is like the tests that were supposedly set up by a secret branch of the military for suppressors. The test in question was very brutal and when one company tested another company’s suppressor in this fashion they were able to destroy it during the legitimate testing. But, when one of theirs was subjected to the same brutal testing, it had it’s own problems.

The point of that anecdote was to point out that a test can go way above and beyond what is going to be realistically expected of a product. Basically, you won’t find a real soldier putting several Beta C mags through his M4 one after the other to the point of destruction and therefore it’s a moot point as to which can survive this test.

12

I dont know why people are SO against the piston guns. I’ve had several DI ARs and now have several piston ARs. I am now behind the piston ARs. They run smoother, cooler, and far cleaner. What is there to complain about? The only problem with most of the piston ARs is price. The only thing I’m worried about is that all the AR companies will rush to put out piston guns and put out some junk. I’m in the military. I think a great fix for all the problems everyone has with the M16 could be fixed with a 12in Barrelled, 6.8mmspc, LWRC upper, 6.8 drop cam’d ACOG with a J-point, and a quick detachable can on a M4 lower.

13

which piston upper was that ?
where can I order SAR online ? did they have the MK18Mod 0 article in the March issue ? thanks.

14

And what DI gun did they use?

15

I have 3-4 thousand through my 16in 556 LWRC upper I bought a year or so ago with zero problems. I shot this one pretty hard, really hard. I’m getting 3/4 to one inch groups with TAP 60gr {My and it’s favorite ammo}. The only time it did anything wierd was when the bolt didnt lock back with one of those super great HK mags. Never repeated. Love it.

16

Points:

The M4,M16 piston or not, is not a SAW.

I think that on actual break out drills in training or actual combat, the Beta C mags would fail before the M4/M16.

17

Ditto.

However I’ve put 14 mags thru a M4A1 making up for a MG that went down in our CAT car.

18

How fast? That was 420 rounds- quite a bit more than the 264 the test rifle puked at.

19

Obviously there are different forces at work here, both physical, and perhaps cynical as well SHZ describes them.

Yes, I find it interesting that these guys made a DGI system puke after only 264 rds. But on the other hand, there were real documented problems with SOCOM smoking through barrels with sustained full auto fire.

I had assumed that a distinguished mag like SAR would not be pushing a political agenda but searching for the truth.

But hate to admit it, SHZ brings up a very valid point, in that this test would have been much more realistic if they had used 30-rd mag dumps. But I’m sure their response would have been that they were testing the gas sysytems not the weapon’s realiability, per se.

20

Thanks for noting the few valid points I was making. I make them very seldom and they are even less seldom noticed. :wink:

Basically those guys recreated a “test” I’ve seen many times where someone knows around how many round it will take to melt a gas tube and that’s all they did.

It’s a magazine, they get their money from somewhere. There’s always an agenda. To most people it doesn’t matter and it’s an article written for entertainment.