AR: Direct Impingement or Gas Piston?

[Coal Dragger]

If we are only talking about AR15 type rifles here, I will take a well built DI rifle/carbine every time. That is how the AR was designed by a rather talented engineer, and then refined by more fairly talented engineers. I am not a huge fan of the AR, but I do own one and other than the crap barrel CMMG put on it there have been no reliability issues. The M16's that I had issued to me in the Marine Corps were also generally quite reliable except when blanks were being used. Blanks are the invention of the devil himself as far as I am concerned.

For those that would like a GP rifle or carbine why try to bastardize one onto a system not designed for it? Why not just go buy a GP rifle or carbine like a SIG 556? From what I have seen they are plenty accurate, and durable and they were designed from the get go as a GP gun.

 

[happygeek]

The previous poster did hit the point. The original question was in an AR platform DI or GP. So G3s, FN FALs, M14s, AKs, G36s, Sig 556s, etc, etc. aren't technically relevent to the narrow question. I suppose a better original question would have been "do you prefer DI or GP rifles?".

 

[Coronach]

Depends on whether or not that was what the Op was trying to ask. I was under the impression that he was asking about DI or GP ARs.

[1] Shot peened, MPI'd AISI 8620 alloy of the dimensions of the bolt carrier tells me that the high torque isn't that big of an issue - for the bolt carrier.

And the bolt?

[2] I've yet to hear reports of actual uneven wear but I don't doubt that there exists evidence. I'm a trained engineer of 20 years and it's obvious that any eccentric loading will result in assymetric wear patterns. Is it an issue? I doubt it.

The problem is that piston driven ARs have only really been popular recently (yes, they existed before). Give it 40 years and lets see what odd wear patterns develop in aluminum uppers from the steel BCG tipping and wearing differently.

[3] The AR-15 already has uneven loading on the bolt lugs. Adding high temperatures to the mechanical cycling can only worsen the problem, no? Yes, I'm correct here.

Sure, but you're assuming that the unbalanced loading is the same across designs.

[4] LMT has a one-piece bolt carrier thus no shearing of keys or bolts.

True enough, but the fact that they had to go to that is evidence that radically different stresses are placed on the internals. Now they've effectively (we hope) dealt with those stresses at this point. What about the next point?

Mike

 

[Aquila]

The bolts are also AISI 8620 and they, too, are shot-peened, High Pressure Tested then MPI'd. The lugs are a bit small and there is eccentric loading onthe lugs opposite the extractor. Eliminating sharp edges and providing generous chamfering of corners eliminate stress concentrations. Witness H&K and LMT's Enhanced Bolts. Of course, if you were starting from ground zero you may wish to enlarge the diameter of the bolt to provide more material strength with the lugs AND provide additional real estate for a more robust extractor design.

I don't think we really need 40 years of shooting to determine if eccentric loading inside the receiver is an issue. Simply take ten (10) uppers and shoot the Hades out of them with a predetermined shooting schedule. Make the schedule aggressive; like full auto every fourth magazine. Replace barrels and lowers as needed. You'll get your answer. You'll get it sooner if every other magazine is full auto.

Whatever the loading on the lugs ADDITIONAL HIGH TEMPERATURES MAKES IT WORSE; FAR WORSE. ALWAYS. EVERY TIME. IT'S PHYSICS/MECHANICS.

It's obvious to me that the carrier key would be subject to repeated IMPACT loadings so the key and bolts would experience far different loading than a DI system. There is the larger impulse and a moment at the base of the carrier key where the bolts fasten it to the carrier. Not surprised about that at all. Making the bolt a one-piece is the way I would have gone, too: eliminate any areas of weakness. Simple stress analysis would take care of that.

The manner in which H&K and LMT put their systems together I'd venture to say that many of the "obvious" concerns have been addressed. The thin op-rod of the LMT may be a concern for some but picking up my college Mechanics of Materials and a Design textbooks alleviates any concerns for me, anyway. It won't see the loads required to buckle it. I like the fact that H&K uses a larger diameter rod nevertheless.

I don't really get the negativity around the gas-piston system. I'm not wedded to ANY machine. Hell, I'm not wedded to this Society. Things are what they are. I didn't write the laws of Physics and I'm bound to them like the next man. Many of us could design, verify, plan, test, manufacture a proper piston system for the AR. It's *NOT* rocket science.

 

[MCMXI]

Like I said, I have no dog in this hunt...I just tend to trust the opinions of folks that have more demonstrated knowledge like Zak or Mike.

No disrespect to Mike or Zak, but I didn't consult with them when I bought my last AR (a POF) and won't be consulting with them when I buy my next one ... most likely a POF in .308. Now if either one of them wants to send me an AR of THEIR choosing then I'll happily provide a shipping address.

Back to the topic at hand ... isn't the AR-18/AR-180 basically a gas piston AR?

 

[Zak Smith]

By all means buy what you think will serve your purposes best.

The AR-180/18 is quite different from the AR-15, however, I believe the bolt lugs are similar.

 

[Funderb]

Because the gas cylinder doesn't trap carbon does it? It's not like the gas piston can't be impeded by built up carbon, right? At least some of the carbon from the DI goes out the ejection port so it does have an "exhaust" of sort unlike the piston system.

any pistol should have vent holes. just look at an sks or AK.
A lot can be said for not making a piston too small.

 

[Maverick223]

The AR-180/18 is quite different from the AR-15, however, I believe the bolt lugs are similar

+1, that (and the designer) is about the only similarity. A AR-18 class has a much beefier bolt carrier, dual (smaller) action springs (allowing folding stock use), a GP system (small tube unlike an AK), longer barrel (Typ.), different magazine catch (although most AR mags are cut for both), and a stamped sheet steel receiver. The system has a proven service record (for the UK) which demonstrates superior dependability and durability vs the AR; though I am not picking on the AR, it has become a decent weapon system after a bit of development/refinement/time.

 

[Coronach]

No disrespect to Mike or Zak, but I didn't consult with them when I bought my last AR (a POF) and won't be consulting with them when I buy my next one ... most likely a POF in .308.

I dont speak for Zak, but I'm just not convinced that the GP is better. I'm also not convinced that it's necessarily significantly worse, though some of the iterations have been breaking earlier than they should. Since I have yet to see an AR jam up from excessive carbon fouling, or anything else one can attribute to the supposed problems that the piston fixes, I'm still scratching my head about what problem the piston solves. this just seems like a lot of money and uncertainty devoted to addressing a non-issue.

Mike

 

[MCMXI]

Mike,
POF's web site seems to make a bigger deal about the heat issue (or lack of it) and the lack of lubrication needed for their uppers to run rather than the fouling issue.

I paid $1100 for my POF upper and it came with a fancy fluted barrel so they're not THAT much more than a quality DI upper. Anyway, it's going to be interesting to see where this top is at in a couple of years.

I don't know about the OP, but you've sure made a lot of good points against retrofitting a GP in a DI upper. I was thinking of doing just that last year but now I'm glad I didn't.

 

[Zak Smith]

After about one magazine of suppressed fire through my Noveske 12" (DI) SBR, all the lube is gone anyway, yet I can shoot it almost indefinitely in this condition.

 

[MCMXI]

After about one magazine of suppressed fire through my Noveske 12" (DI) SBR, all the lube is gone anyway, yet I can shoot it almost indefinitely in this condition.

Only ALMOST indefinitely!! The one in this review ran for 16,000 rounds without any cleaning or failure, and then ran another 3000 without any failures. That's a good sign right?

http://www.pof-usa.com/GW_POF_P415_[1].pdf

This has been interesting but at the end of the day there will always be some for and some against ... I'm just happy that we have a choice.

 

[45B@cav]

My turn, I have carried the M4 for 15 months in Iraq. I have a Combat Action Badge to prove that I actually used it on 3 yes I said 3 occastions. The only part of the whole gun that ever saw any form of lube was the bolt. Not the carrier just the bolt. I personally as well as a few other vets can say they owe thier lives to Stoners design. It fired flawlessly and accurately every time it was called on. I own 3 DI AR's and would not ever think about converting them. I own 2 AK's so don't think I'm biased. If I had to pick a rifle to carry into combat it would be a DI Stoner design. As for any comments on Reed Knight, I don't like the SOB but I respect his OPPINION!!!!!!!!!!!!!! This thread is a matter of what you like. Lazy F#$% or finatic gun polisher, I don't care at all. The DI works clean dirty or in between. It was designed to. Do you actually think Eugene Stoner thought it would never get dirty. It is designed to work with that carbon and s#$% in it. I've seen M16 varients at ranges with crap growing in the chamber that looked like dust bunnies on crack and they shot 40 UP 40 DOWN NO HICCUPS AT ALL. As for the SF guy's you talk about I have some very very good friends in 10th group out of Colorado and the oppinions I got where far far less than stellar on the almighty HK416 or HK41sh$% as I remember it being referred to. Can't touch the accuracy of an AR ON IT'S BEST DAY. Oh yeah Aluminium disapates heat alot faster than steel. So I will say that in a certain cover and run situation I was involed in I personally dumped 6 30rd mags as fast as I can reload and to all you CARBINE CLASS PEOPLE! That is really fast. The weapon never got to hot to hold with bare hands. I know because my gloves had a hole in them. I think it's great that people take these coarses to better thier ability to defend what is thiers or be better armed citizens but I personally am tired of the people that think they will win any gun fight because they took a CARBINE COARSE. The fact is NOTHING IN YOUR IMAGINATION CAN EVER PREPARE YOU FOR THE REAL THING! The coarse is not a waste of money, I think they are great tools and I would like to teach them one day. It is very different when bullets are coming your way though. I think it is very responsible for any gun owner to have some form of formal training but don't think you are the s&^% because you took some class. If you have what it takes to rise above in that moment of terror, you will. The class is just a little reassurance of what is already inside. The human animal is a survivor. Our body is more frail then most but our will to survive has got us this far. Iam happy as a pig in s@#$ with DI but as a survivor I'd use foul language and sharp sticks to get the job done. I hope none of you ever have to feel the real thing GOD BLESS THOSE IN HARMS WAY FIGHTING WITH DI RIFLES SO YOU CAN BITCH ON THIS FORUM!

 

[GBExpat]

Paragraphs, 45B@cav, paragraphs ... such a mass of text is much easier to read if it is broken up into smaller chunks ...

... and Thank You for your Service to our nation.

 

[Zak Smith]

Only ALMOST indefinitely!

Please read my post more carefully-- the study you quoted says nothing about shooting suppressed for 16,000 rounds. The point of my post was about "lack of lubrication needed for their uppers to run".

 

[MCMXI]

Zak,
I read ALL 29 "words" in your post carefully ... my comment was tongue in cheek because I realize that this thread is going nowhere. I was happy to leave it there but now I'm not so sure. You obviously think that GP uppers are CRAP and that they offer nothing in terms of benefit ... you've made that perfectly clear on numerous occasions. You might as well come out and state that POF is ripping people off by suckering them into spending lots of money for an inferior or redundant system, in fact, why not just state that for the record? I haven't made any statement that one system is better in the real world since I really don't know, but the truth is, neither do you because there's no way that you've run enough tests, under enough conditions, with enough systems to make a conclusive statement. I need more than vague, anecdotal statements such as "all the lube is gone anyway, yet I can shoot it almost indefinitely in this condition" to be convinced that POF's hard chrome plated parts and silicon nickel surfaces are a gimmick.

I'm fortunate in that I work with and have access to some brilliant individuals at Natick Labs and Picatinny Arsenal (and ARL at the Aberdeen Proving Grounds) who are directly involved in the development/selection/testing/deployment of weapons systems. I'm currently working on a project with an individual who's sole responsibility is to prepare the SEALs and Rangers with the best equipment possible. If I have questions on material selection or suitability I can ask my boss who has a PhD in mechanical engineering from MIT. If I have an optics-related question I can ask my brother who has a PhD in Astrophysics and has published numerous papers in Nature, Scientific American and other top journals. I can consult with a PhD chemist who works in our group developing novel corrosion resistant coatings. If I have a shooting-related question I have access to a US Olympic pistol shooter who competed in Beijing, and an international/national competitive shooter (rifle and pistol) who's won matches all over the world (PALMA too) and who shot on BOTH the Marines and Army shooting teams. He's now responsible for the training of Marines in all aspects of shooting and the scores have greatly improved since he took over the program. If I have tactical questions I can ask a retired Navy SEAL who spent a lot of time working with my wife on her firearms proficiency. So as shocking as it may seem, THR is just a small (but interesting) part of the whole picture.

I have no time for anyone that considers themselves to be beyond reproach or infallible. No one on this board is qualified to make a definitive statement as to which is better under real-world conditions so what we end up with is conjecture, speculation, snippets of "expert" testimony, anecdotal accounts etc. There's nothing wrong with that in a discussion forum such as this. In fact, I enjoy these discussions for what they are ... a bunch of guys sitting around a campfire talking a bunch of BS. I come here for the fertilizer, not the flowers!!!

Now I really am done with this interesting but pointless thread.

 

[Zak Smith]

1858,

False and specious arguments against DI and for GP do not help progress, they hinder it through bad information.

Reliable knowledge is gained through data, careful reasoning, and focused, defensible thesis statements. People have a tendency to want to oversimplify in order to make sense of things with less thinking: X good, Y bad or what is the best handgun ever? However, this is not really a good way to make good decision, or build up reliable knowledge. Things are usually more complicated than that.

In this case, it is erroneous to conclude that I "think GP uppers are CRAP" based on my objections in this thread, which have been more about claims made about GP and DI systems by the GP proponents than the merits GP or DI systems themselves.

-z

 

[Maverick223]

Now I really am done with this interesting but pointless thread.

Good now maybe we can have some paragraphs...

 

[MCMXI]

Reliable knowledge is gained through data, careful reasoning, and focused, defensible thesis statements.

I guess I'm not done (but I will use paragraphs and I think this if the first in over 1000 posts where my grammar or formatting was called into question ...well done Maverick223 )

True ... and so far this thread hasn't provided any, let alone enough data to make a carefully reasoned decision. So anyone stating that "gas pistons are a solution looking for a problem" is oversimplifying a complicated issue.

Your experience with a separated piston on your ONE gas piston upper is only significant to you. Your statement that "all the lube is gone anyway, yet I can shoot it almost indefinitely in this condition." is totally unscientific.

How many rounds did you fire before you couldn't shoot it in that condition? Did you take lubricity measurements of the surfaces of the moving parts before, during and after? Did you measure the bearing surfaces or examine them using an SEM to see what kind of wear was occurring under those conditions or what stress risers were being induced under those conditions? Did you take temperature measurements of the moving parts to determine if they were heated enough to change their mechanical properties or to induce high temperature creep?

I have a BS and MS in mechanical engineering and have worked as a ME for the last 9 years along with working as coded welder/diver for over 12 years but I know enough to know that I don't know enough and that it's a complicated world. Anyone who thinks they have ALL the answers is delusional and quite possibly dangerous.

 

[Zak Smith]

While I agree that this thread hasn't provided much if any useful information, it has provided an opportunity for some claims of GP advantages to be challenged. Challenging an argument with counter-arguments or data is an important part of getting better knowledge.

Your experience with a separated piston on your ONE gas piston upper is only significant to you.

The M96 isn't a gas piston upper, but the occurrence is relevant to split piston designs.

How many rounds did you fire before you couldn't shoot it in that condition? How .... ?

That upper has never failed to run, regardless, so I couldn't say.

 

[Aquila]

I really don't see a challenge, Zak, et als. No other modern design incorporates a Direct Impingement system. Besides being able to have the rifle lighter in weight and slightly less reciprocating mass I see no advantages of the DI from an engineering perspective.

 

[MCMXI]

The M96 isn't a gas piston upper, but the occurrence is relevant to split piston designs.

But that's just one data point. Maybe the separation was due to a manufacturing defect rather than the design itself. Maybe the piston wasn't heat-treated properly ... who knows? Maybe that particular manufacturer tried to cut costs by buying a load of cheap raw material from China! If this is occurring on numerous gas piston systems from numerous manufacturers under numerous operating conditions then maybe there's a trend but as it stands it's meaningless.

Like iamkris keeps saying "I have no dog in this fight" since my life (currently) doesn't depend on the reliability of either system that I own. I have the luxury of enjoying both and I do. However, I'd sure like to see some extensive, objective test results comparing both systems but I doubt that's going to happen on this forum.

 

[Maverick223]

well done Maverick223

Thanks, 1858

 

[Maverick223]

I have a BS and MS in mechanical engineering and have worked as a ME for the last 9 years along with working as coded welder/diver for over 12 years

I too am an engineer, full of BS, with a BS (Civil)...

but I know enough to know that I don't know enough and that it's a complicated world

I like this statement , Although I am a CE, I know a great deal about materials (I work for a Forensic Engineering Firm) and mechanics (I see plenty of things move that shouldn't ), however I do not profess to know everything about any weapon, especially one with so many iterations.

 

[gga357]

If I win the lotto I will buy 6 each and fire 25,000 rounds througheach on a predetermined schedule and take measurments/accuracy at 0, 500, 1000, 2000, 5000, 10k, 15k, 20k and 25k.