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AR Operating Systems: Gas Impingement vs. Piston

Can the gas-piston AR system overtake the longest serving military rifle, or will the gas impingement system soldier on?

Gun owners have strong opinions when it comes to Eugene Stoner’s AR design. There are those who decry the path of the “new and improved” gas piston guns as mechanically unsound and inherently flawed. On the opposite side are those who deride the traditional AR operating system as a poor design, self-fouling and jam-prone. Standing in the middle of the fork is everyone else, the undecided, trying to determine which way to turn.

The first and most important reference mark is the fact that the Stoner-designed AR with a direct gas impingement system is by far the longest serving military service rifle in U.S. history, but its reign has not gone unchallenged.

In 1986, the Army mounted the Advanced Combat Rifle (ACR) project to look into replacing the AR. Heckler & Koch proposed a caseless ammo gun while Colt, McDonnell Douglas, Steyr and AAI Corporation came up with a variety of candidates that didn’t meet the Army’s needs. The government spent $300 million on the ACR program without a useable replacement.

Picking up the pieces of the ACR program was the Objective Individual Combat Weapon program (OICW) which led to HK’s XM8. The XM8 included a 20 mm grenade firing module along with a 5.56 NATO HK G36 variant as its kinetic component. The OICW XM8 was all kinds of cool to look at it, but it was plagued with cost, weight and functionality problems and resulted in another goose egg for the U.S. military, as did the XM8 as a stand alone platform.

The most recent military rifle program was the SCAR (Special Operations Combat Assault Rifle) run by the Special Operations Command (USSOCOM). SCAR’s scope was far less ambitious than either the ACR or OICW programs, which were “Big Army” projects to replace the M16. The SCAR program was limited to fielding a rifle for Special Operations forces, which have a separate budget. The winner of the SCAR program was FN Herstal, the legendary Belgian manufacturer that produced so many of John Browning’s designs.

And yet, in spite of these highly publicized attempts to replace the AR, none have completely succeeded. Either the entire small arms industry is incapable of bettering the AR, or the Army imposed such requirements that none could meet them. It’s probably a combination of the two, but the fact remains that the basic AR designed by Eugene Stoner in the 1950s continues to soldier on.

Operating Systems
The AR is a self-loading rifle that performs a basic set of functions without manual assistance from the operator. After the trigger is pressed, the gun must fire a cartridge, extract the fired case, eject it, pick up a fresh cartridge and transfer it from the magazine into the chamber, lock the breech and cock the hammer (or striker) to return the rifle to battery—a round in the chamber, ready to fire with another press of the trigger.

It’s really a straightforward mechanical operation. The best and brightest firearms designers have achieved it for the past 120 years with a variety of ingenious solutions.

Two of those solutions are the direct gas impingement system and the short stroke gas piston system. Eugene Stoner utilized the impingement system in the AR. It works by bleeding propellant gases through a port at the end of the barrel and channeling the gases back through a tube to directly strike, or impinge, a bolt carrier, thereby pushing it rearward to extract and eject the fired case and, as it’s propelled forward by a spring, to strip a fresh round and push it into the barrel’s chamber.

A short stroke gas piston system is what Mikhail Kalashnikov used on his AK-47. The piston system also relies on propellant gases that are bled through a small hole in the barrel, but instead of the gases traveling through a tube to impact a bolt carrier, the gases are contained in a cylinder in which there is a piston, like in a car. The gases push the piston, which in turn is connected by a rod to a bolt carrier that moves rearward to extract and eject the fired case and, moving forward from spring pressure, strip a fresh round from a magazine, chamber it and lock into battery.

What’s causing a fork in the AR road right now is that a number of manufacturers have decided to modify the Stoner design to operate with a piston system instead of an impingement system. The question before the house is: Do we need to fix the AR with a new operating system and, if so, do the new piston systems achieve that remedy?


An Answer In Search Of A Question
The sole claim to fame of a piston system is that it’s more reliable than an impingement system. The reason given is that hot, dirty gases are not spewed into the action of the rifle like the direct impingement system, fouling itself with heat and carbon, depositing black crud all over the bolt carrier. Instead, the gases are contained in a gas cylinder which is self-cleaning.

No one asserts that piston systems are more accurate or more durable, just that they’re more reliable because the bolt carrier is not caked with fouling and subjected to scalding heat.

Advocates of the piston system are quick to ask, “Why would you dump hot, dirty gases where your rifle feeds?”

There’s no question that heat and fouling are highly detrimental to moving parts in a firearm. The solution, however, does not necessarily require redesigning the whole gun. Fouling problems can be avoided quite easily with a marvelous little thing called lubrication.

“Keep her wet. That’s how you run an AR,” a hard-bitten range master once told me. “I don’t care what you squirt in there—BreakFree, WD-40, lime juice. It doesn’t matter. Soak her good.”

A good friend of mine, Sgt. Jason Davis of the Arcadia (Calif.) Police Department, has an M4 carbine with more than 2,500 rounds through it without cleaning. “I just keep it lubed,” he said. “I run it with a suppressor too, so it gets even dirtier. I’ve never had a malfunction, not one.”

Davis is not a glutton for punishing his gun. His no-cleaning test came about by chance.

“At first, I just didn’t get around to cleaning it, but after awhile, I realized I had a sort of torture test going without intending to. I wondered how long I could go without cleaning my M4, so I made a point of logging the rounds and just didn’t clean the gun. I lube it up before every session,” the lawman said.

I’ve never gone that far, but I’ve fired 1,500 rounds without cleaning, but I keep my bolt carrier wet. Reliability is not an issue with liberal lubrication.

Accordingly, it’s my opinion based on personal experience and a lot of anecdotal evidence that Eugene Stoner’s gas impingement system works as advertised. The piston system guns are, as the late Col. Jeff Cooper liked to say, “An ingenious solution to a nonexistent problem.”

Unintended Consequences
Even if you take the argument of the piston system at face value—that it’s more reliable—you still have the law of unintended consequences to deal with. First, piston guns generate more felt recoil than impingement guns (although that’s not a huge detriment since we’re talking about a 5.56 mm here, a “poodle shooter” as Col. Cooper sniffed).

More importantly, however, a piston system alters the mechanics and timing of an AR in a manner that a growing number of shooters are claiming is harmful to the gun.

There are new systems being developed, tested and marketed now, but generally the problem is that a piston system is attempting to retroactively adapt a bolt carrier that was designed to function with direct impingement.

What we’re seeing are piston systems substituted for the gas tube of an impingement system by simply inserting a piston into the mechanism. The same buffer system is used to return the bolt carrier into battery, the same geometry of the bolt carrier is utilized and the same timing of the cycle rate is retained.

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41 Responses to AR Operating Systems: Gas Impingement vs. Piston

corriedewilde wrote:
September 30, 2014

I am of the opinion that the AK's reliability has to do with the loose tolerances. It is run completely dry in RSA's sandy, semi desert conditions, any oil will have it jammed very quickly. I have seen the same the same with FN FAL's, well worn para-rifles run dry, do not have a problem. Currently the R4 (copy of Galil) is beautiful engineerd 5.56mm AKs, but need to be cleaned continually or they jam due to dirt collecting on the oil.

Freq18Hz wrote:
May 02, 2014

Frog Lube? SEAL teams take it on diving expeditions? How many seals do you know? Froglube is soy/water based. It's for cold climates where oils freeze/become viscous.

Archangel wrote:
January 06, 2014

Hey Dave....If 'wet lube attracts dirt dust, sand, other environmental crap', then Use DRY Lube instead. I know it's not 'tacticool' but Problem solved Frog Lube adds another alternative, and it is frequently used by SEAL Teams who take their M-4s on diving expeditions. :)

JCitizen wrote:
November 18, 2013

I got to admit, trying to convert a DGI system to piston is probably folly; but Stoner also designed the AR-18, which he considered superior, and kept the copyright so he could manufacture it. I weighs the same as the AR-16, shoots just as straight - I challenge anyone to tell me the recoil feels any different. I've had pack rats try to put nesting material in the receiver and not affect the ability to operate the action.(purely accidental field experience) The big difference, is I can break it down in seconds, and have the piston squeaky clean in way less time than the old Colt system. It is those seconds that count in combat - not the stupid gas system.

Benjen Smith wrote:
October 23, 2013

Delta uses a Gas-Piston system (i.e. HK 416) over DI (i.e. M4). Enough said in regards to reliability and accuracy.

TripodXL wrote:
October 07, 2013

Everything has its purpose. DIGS saves weight and allows a more accurate rifle, generally. Pistons 'can' offer better reliability, but they do so at greater recoil Impulse (less accurate) and higher parts count (i.e. higher failure rate eventually). So the solution for the piston rifle is to clean your rifle religiously and to replace the piston parts at the first sign of wear or at specified intervals to prevent.......WAIT A MINUTE, I think I have heard this before! Hhhhmmmn? When a DIGS rifle goes down it is typically (and I only know this anecdotally as I have never had it happen) either dirty and/or dry. YOU ONLY GET WHAT YOU PAY FOR, whether it is $ for a piston system or 'sweat' for keeping your rifle clean and lubricated as you should. In any event a good cleaning or if need be a good hosing out and wet, wet, wet, lubrication will fix it. If a piston rifle goes down...well cleaning isn't usually gonna fix that. It is broken...been there and done that. Short version; was drafted in 72 and retired in 10 and NEVER had, nor have I ever seen an M16/M4 variant service rifle that failed unless broken, INCREDIBLY dirty/dry or had mag problems (all firearms that are magazine fed can suffer from that problem). No weapon system can endure not being cleaned indefinitely (and don't even mention AKs, got really tired of teaching Iraqis and 'Stanis' how to clean their POSs so they would work) and continue to work and give you a high level of confidence. My son was in Ranger Btn (tab and scroll w/ 7 tours) and I happened to be visiting and I asked about 25 of them if they had the chance to pick up an AR or an AK what would they choose. No one mentioned AK. These were all E-4s and above that had CIBs and multiple tours and plenty of 'training' on AKs. They weren't remotely interested. If cleaned and lubricated an AR will not fail you anymore than any other weapon. I always carried the small can of WD-40 and CLP next to my blow out kit when 'out and about'. That is how you keep it running. Be well.

chico wrote:
August 06, 2013

I have been in the army for 11 yrs and never had problem with gas operating systems. I have seen a broken piston before though...take care of ur equipment and it will take care of you...in even in all of the harsh weather conditions...

Dave wrote:
June 08, 2013

The most important thing that this article does not mention is how HOT the bolt gets on the impingement system. When you have a round in the chamber, it can cook off. While it is rare, it HAS happened. Also, the heat causes a viscosity breakdown similar to and engine that overheats. The author rightfully mentioned to keep the gun lubed to prevent viscosity breakdown and part wear, but excessive lube creates a whole new problem. Lube collects dust, sand and dirt. While the impingement systems don't necessarily jam from carbon, heat and gas, they DO JAM when they collect too much dust, dirt and sand - like those found in most combat environments (i.e. woods, jungle, desert). This is why most law enforcement don't complain about jams with their impingement guns, because they are not exposes to those environments. Anyway, I hope my comments helped to add value to the discussion.

TimB wrote:
June 05, 2013

Joakley....the Sig 556 IS a piston gun.

Clay collier wrote:
May 12, 2013

I wish i could get my hands on an AR15 that shoots like my HK 91! WHATS THE CLOSEST THING OUT THERE? ( I do love piston operated firearms, they seem to be great shooting as well ) ..

Veteran Ro wrote:
May 07, 2013

Agree to comments when using the Piston system in a Combat environment (Micah) - you get a little extra 'comfort room' for cleaning and fouling. I was in the Gulf, and some other places that challenged my Weapon.....and I was a SAW Gunner....so I too felt it was better and more reliable. I made sure to clean my Weapons (m4, SAW M249). In a Civilian Environment, Hunting, Home Security, or LEO - the Gas Impingement is just fine. Never had one problem if she is kept clean and lubed.

Daniel wrote:
April 07, 2013

Under extreme combat situations, gas piston rifles have proven themselfs. Look beyond the ak, look at the FN, its legendary.

glocksigfnh wrote:
February 23, 2013

Re: Joakley The Sig 556 series of rifles and pistols have adjustable piston operating systems base on the Swiss Arms 551 rifle. The 516 and 716 rifles also use an adjustable piston operating system, but maintain the look, feel and function of an AR-style rifle.

Joakley wrote:
January 27, 2013

The Sig 556 is a DI, its the Sig 516 and 716 that are Pistons

Micah Covington wrote:
January 15, 2013

One thing people seem not to realize about military rifle testing trials, is that the makers of m16/m4 rifles pay a lot of money to make sure their designs are used. There are many rifles with better designs than m16/m4s. I carried a m16 in Iraq and was a unit armorer. The M8 was a much better weapon. M249's and M240B are both piston driven. I was a 240B gunner in Iraq. No failures to fire, ever. Granted, if you clean your M16/M4 it will work almost as well, but the piston system gives you a little more leeway in how clean your weapon is. Keep in mind that this was desert fighting. Fine sand in the summer and mud in the winter. Even if you didn't fire your weapon on a mission, it was still dirty when you came back.

michael wrote:
November 24, 2012

Use the powder the gun was designed for and it will run fine.My prob with DI is gas rings,keep plenty of spares or you're out of bussiness.My piston AR uses no gas rings and uses any mill spec bolt.

michael wrote:
November 24, 2012

I guess a gun that can't shoot all calibers is unreliable too?My car has 12/1 compression runs fine on super unld designed for it.

Dwight Pilkilton wrote:
November 10, 2012

Mikhail Kalashnikov did noting original, he copied Carbine Williams design, he also copied the safety...take a look at a Model 8 Remington and look closely at the safety.

sp101man wrote:
October 23, 2012

Mike, both Ransom and Panzer SS are correct. The HK91 does not rely on propellant gases in any way. It is unaffected by burn rate, power of the load, or residue. It uses a delayed roller block action.

Ransom wrote:
October 07, 2012

Mike, The HK91 uses a roller bolt delay NOT a gas system. Not sure what rifle you are looking at but both of mine lack a gas system. BTW, 100% reliable.

Mike wrote:
September 27, 2012

Uh, PanzerSS, the HK91 most certainly does have a gas tube and a piston. The piston is attached directly to the bolt through an operating rod which rides in the gas tube that sits on top of the barrel. What, exactly, do you think causes that rifle to function?

PanzerSS wrote:
September 16, 2012

Get a HK91 and you don't have worry about either. No gas tube or gas piston there.

gt762 wrote:
September 12, 2012

A gun that is picky about the type of gunpowder it burns from its cartridges is not reliable... if your car could only run with one particular brand of gas would you call it reliable?

Paul List wrote:
July 17, 2012

history says this;If the AR had been able to use the ammo it was designed to shoot. Witch it was not.Us had a contract with Olin for powder to power the ammo of the M-14 oh is that a gas impingement rifle? And so to keep out of a law suit US decided to down load the ammo or allow ammo not designed for the AR to be manufactured.So now we have dirty ammo only because US shortcut to keep out of court. Call that strike 1. Then the US knows from experince all about chromed chambers and bore.Ignored to promote speed of build and $$. Stike 2. This is the equivilant of substituting kerosene for high test gas and not making nessasery changes in system to acomodate the change.The M1 rifle & cabine,the M14 and countless other guns all have proven to perform superbly, or nearly as any of the direct blowback systems.

David wrote:
March 28, 2012

I own a DPMS SASS(AR-10) .308 and a SCAR 17s chambered in .308. No, they're not .223's, but both are great weapons to shoot. BUT, if I had to compare I would take the SCAR hands down. The ease of operation, disassembly, lack of fouling, and low recoil make it an easy choice. Every person, and I mean every single person that have shot both rifles back to back prefer the SCAR. Don't get me wrong, the AR platform is a tried and true platform and I enjoy my AR, but it is outdated and needs improvement to meet the competition head-on. The M-14 was was preferred in Vietnam over the M-16 by all soldiers who could aquire one, and that is definitely not new news. Gas piston AR's do have some advantages over their direct impingement brethren, however, starting ground-up would make the most sense so that the latest advancements can be utilized in the design to help the soldiers in the field and the civilians at the range.

Dave wrote:
March 04, 2012

The argument of "keeping it wet" is misleading. The carbon deposits are simply being redirected to some other location in the mechanism to collect over time. You'll still pay the piper and it will likely cost more in time required to truly clean out the system. Avoiding the deposits in the first place is the best way, IMO.

Josh wrote:
February 20, 2012

Correct me if I'm wrong but doesn't a DI AR just damage your bolt over time, and bolt assembly's cost a fraction of what your paying in extra $ for a piston AR? If it ain't broke don't fix it IMO

Dan wrote:
February 19, 2012

the anti-piston people are just wrong period. some of you talk about respecting your weapon blah blah blah. the military has tested these weapons time and time again and the gas impingement looses everytime! the genrals in charge simply retort "98% is good enough" unbelievable that we send our troopps out with not 2nd or 3rd best but 4th! since Iraq and Afghanistan the military is finally starting to recognize the poor reliability in dirty environments. Many of you are end of the world conspiracy people so if this is what you belive you better get yourself a piston impingement or find yourself loosing to those of us that do have them. by the way, the pisten system is older than the the AR DGI.

Medic wrote:
January 15, 2012

The gas entering the bolt carier puts space between the carier and bolt. This force is inline with the barrel and recoil system.The rings on the bolt help seal in the gas, and the two holes in the carier vent these gases to atmosphere.Rings and vent holes all in line with the barrel. This is not the same geometry as the piston systen that exerts its force only to the top of the carier.

Larry wrote:
December 11, 2011

The piston system didn't work well in the M1 Garand? In the M-14? Remington 1100 shotgun et al?

Jay wrote:
September 27, 2011

AKs do not use a short stroke gas piston they use a long stroke gas piston. The SKS and SVD however uses short stroke systems.

Wulfkin wrote:
September 13, 2011

Check out Adcor Bear no carrier tilt and free floated barrel and no hot gas in your eye, all for around $1000 now is there a reason why NOT to get a piston?

PK wrote:
August 21, 2011

If your intention in getting a piston system is so that you won't have to clean it, don't get a gun at all. Respect your gear. I'm also not buying the "WD-40" as an answer to the impingement system dilemma of fouling the action. Hard to swallow the anti-piston argument in light of the AK success and durability. However, just sticking a piston in a gas tube sounds heretical and . . . stupid. It's unfortunate that the AR18 wasn't embraced as it could have been. The SIG is a piston system.

D wrote:
August 20, 2011

I might be wrong, but i'm pretty sure that the AR18 didn't use a gas impingment system because Armalite had sold the patents over to colt. At the time of the design of the AR18 Stoner had left Armalite and had no involvement in its design. Also Stoner preferred the impingement system over the piston.

JK wrote:
August 05, 2011

Uhhhh..... Nobody wants to mention the AR18... Stoner himself intended to "improve" the AR15 with a gas piston system.

V wrote:
July 27, 2011

FERFRANS Weapon System. Google it, Yahoo it, YouTube it, search for it.

blue wrote:
July 27, 2011


albsr wrote:
July 24, 2011

for those of us that are still looking at the AR type I would like to know the maker of the "one piston gun that has beaten time after time"

V wrote:
July 19, 2011

All things evolve in life. Going to the moon was a dream and then became reality and so on in life. It would be a moronic to think that the AR platform can not evolve past a design made how many years ago? Not all piston guns are created equal. I have observed most of the brand name piston guns break (not going to mention names) but there is one piston gun out there that has beaten time after time every other AR made. Do your home work and keep an open mind (Chevy or Ford).... Oh brother here we go again. (if there are miss spelled works sorry, hard to comment from iphone)

lbsrdi wrote:
March 08, 2011

I believe my SIG 556 SWAT is not an impingment system. Am I correct?

Parasome wrote:
February 26, 2011

Gas piston system to many moving parts. In addition a weapon should be maintained and respected not neglected.