AR-15 Pistons: Pros & Cons


So, should you go AR-15 piston or not? That depends. One group who benefits greatly from a piston system is those who own suppressors. The delayed gas flow (that’s how a suppressor works: it delays the gas flow out the muzzle, to reduce noise) means more gas and gunk blown back into the receiver on a DI rifle. Depending on minor variables in each rifle, running with a “can” can mean a gun that looks like a 4th of July charcoal grill after a few magazines, or simply a more-difficult cleaning job after a day of shooting. An AR-15 piston system on a rifle with a suppressor (especially a piston system with an adjustable flow valve) can make shooting with a “can” a pleasant time.

Another group that finds favor with piston systems are those with SBRs. The short-barreled rifle crowd often finds that a short-barrel DI system is just too touchy, or in order to be reliable, has to run too violently. Let’s take a look at the math involved.

Our bullet screams past the gas port, and thus allows gas to flow into the system. The bullet continues onward, and the system remains sealed until the bullet leaves the muzzle. How long is that? The time period is called the “gas dwell time,” by the way. Well, on a twenty-inch rifle, we have a 55-grain FMJ bullet leaving the muzzle at some 3200 fps. That means that the distance from the gas port to the muzzle, some 6.5 inches, produces 0.00017seconds of sealed-bore gas dwell time. (Those who know their mathematics realize that it is not simple arithmetic, but a calculus application, but I’m fine with rounding the numbers for this demonstration.) So, .17 milliseconds of time, which is less than the duration of a typical camera flash at its peak.

On a CAR with a 16-inch barrel, that dwell time is .24milliseconds, an improvement, but from there it goes backwards. TheM4, with its 14.5-inch barrel, gives us .19 milliseconds, and an 11.5-inch SBR produces a miniscule .11-millisecond dwell time. To ensure that the short-barreled rifle works, you have to open the port to get more of the gas working for you.

AR-15 piston systems are much less touchy. You see, you can hammer the system with as much gas as you need, and use a built-in gas bleed, or a self-limiting piston, to control over-driving it. Use a piston system and the SBR becomes a far less touchy creature, working with a wider array of ammunition, bullet weights and loads, and doing so with greater reliability.

So, those of you with SBRs may find a piston system advantageous. The rest of us? Not so much.

Finally, cost. Part of the cost of an AR-15 piston system is the piston system itself and the R&D that went into developing it, as well as the tooling costs to fabricate the piston parts. However, a fondness for the good old days clouds the issue. There are still a number of shooters who remember fondly the $600 AR they bought “back when [fill in the blank].” Inflation aside, let’s look at the“$600 AR” they bought. It probably had plain plastic handguards, maybe the A2/M4 type, maybe not. They certainly weren’t railed, free-float hand guards. The stock was an A1, an A2, or an old-style CAR stock. Not one that holds batteries or offers a solid cheek weld.

The sights were either A1 or A2, no flattop, and no place to mount a scope except in the carry handle. Which sucked. And the barrel? Maybe it was a 1:12 twist “pencil” barrel, and maybe it was something heavier. But wasn’t the premium tube we now expect, in this age of the sub-MOA AR. In fact, it wasn’t a rifle many of today’s shooters would pay $600 for, and that is with less-valuable inflated dollars. Adjusted for inflation, that 1986 AR you paid $600 for would run you $1,186.54 in Obama dollars.

So, before you go grumbling about “how expensive ARs have gotten, “consider what it takes to upgrade the $600/$1187 AR with a new stock, railed hand guard, better barrel and flat-top upper. All of a sudden, an “expensive” AR doesn’t seem so bad, does it? Throw in an AR-15 piston system and they are almost reasonable.

So, go to a piston if you want. Stick with a direct impingement if you want. Add all the features you want or don’t want, but don’t grumble about the cost. For what we get today, the AR has never been a better deal.


Read More about AR-15 Pistons and Other Features

Read more about AR-15 pistons and other featuresPatrick Sweeney is Gun Digest‘s resident AR-15 expert. Read all four of his books in the Gun Digest Book of the AR-15 Series Set. The set contains volumes one through four of Sweeney’s Gun Digest Book of the AR-15 at a much better price than if purchased separately.

Click here to get the Gun Digest Book of the AR-15 Series Set.


4 thoughts on “AR-15 Pistons: Pros & Cons

  1. Kent

    If memory serves, the only other production DI rifle was the French MAS of the 1940’s. The M16 happened twenty years later. I believe that was the only DI rifle Eugene Stoner designed (consider the AR10 family), and no one else has used the DI system since. Stoner went on to design the AR18 and Stoner 63, both piston systems. That said, you would think that if the DI system rifle was all that great, more gun designers would use it, but none did, not even Stoner. That is conclusive.

    About accuracy, I have read much conjecture, speculation, and common knowledge, but I have not seen any hard data generated by the scientific method. If the piston system lacked in accuracy compared to the DI, I’d wager that if the pistons got as much attention, tweaking, and tuning as the DI originally did, either system could win it all on any given day, given the right shooter.

    As for me, I started with a piston AR and never looked back. Cleaning time is better spent shooting, and I have no accuracy complaints.

  2. bhp0

    I have personally used some very high grade AR15 guns in competition. I would never use this type of gas system for a serious survival rifle. Other military guns that use this system like the Rashid, Swedish Ljungman etc. all have the same problem, they jam up when not squeaky clean and drowning in lubricants period.

    The AR15 was a failure in Vietnam and 40 some years later failed in Iraq as well. Remember the cute little blond who was the only soldier among her group that had an AR that kept on working? I might add even a fine mist of water like a light rain will mix with the burnt power and jam up these guns right quick. Its amazing the U.S. is still using such a worthless weapon in combat.

    1. Icorps1970

      This is complete BS. Have you ever BEEN to VN? Have ever carried an AR in the wet and in the dry in the mud and in the dust and grit. Like every day for months? When it HAD to work?
      Both from my experience in VN, where it was not a failure except when willfully sabotaged by using a propellant (after all testing and the rifle was accepted) that they were TOLD by COLT would produce malfunctions. It does not have a problem currently, this from my son who just got out the the USMC and will tell you the same thing. He shot his when you could feel and hear the sad in the weapon and if kept running. I shot my M16A1 one night until over an inch of the gas tube was burned permanently black. It never missed a beat. In fact it never missed a beat in VN EXCEPT when I was issued a faulty magazine. I found this out when not in contact and removed the ammo and mashed it flat.
      If someones AR quits its invariably THEIR fault OR something breaks, not usual with some of the lower end versions out there. Sand and dirt will not stop an AR any faster IF as fast as AK. The gas piston is something someone cooked up to SELL to people who are convinced the AR needs to be more like a AK. The AR platform will easily stay on a human to about 600 yards and a GOOD one on the human HEAD. The AK will not. But all the arm chair experts love to tell everyone what a POS the AR is. I disagree.
      1st Bn/61st inf. RVN 1970-71.

  3. jdmcomp

    I think that the mass in motion in a piston design also need to be considered. Slow motion film of an AK being shot show lots of flex in the parts due in part to the large mass of the piston/bolt carrier assembly. I agree with your conclusion, and find the original design near perfect. All guns need cleaning, so that is never a reason to switch to a piston.