AR-15 Pistons: Pros & Cons

AR-15 piston pros and cons

AR-15 piston systems do not make gas go away. They merely vent it in different locations. Here, the gas is venting directly behind the front sight, as it is meant to.

Should one go with an AR-15 piston or direct impingement (DI) system?

The big drawback to the Direct Impingement (DI) system is the gas blown back into the receiver. It does, however, have several manifest advantages, advantages you should not discard simply because all your buddies say you should.

First of all, it is light. All the system needs is a hollow tube leading from the gas port back to the receiver. Unless you make your AR-15 piston system out ofunobtainium, it isn’t going to be that light, not ever. When you are laden with a whole lot of gear, lighter becomes very attractive.

Also, the hollow tube does not press on or bind the barrel, and so the barrel is essentially free-floated. If you use a free-float hand guard, secured to the receiver at the barrel nut (and to the barrel not at all), the barrel is free-floated, and you can thus wring all the potential accuracy out of it that it has.

With a good barrel, an AR can be as accurate as a lovingly-blueprinted bolt gun.

The AR-15 piston system removes all those advantages. First, it adds weight. Granted, some systems not so much, but they all add something.

Second, part of the weight is a more secure (and often heavier) gizmo bolted on the barrel up front. That weight makes the barrel harmonics of firing a different thing than the DI system. You see, every time you fire, your barrel gets hit as if by hammer. It vibrates. Accuracy is the bullet leaving the muzzle at the same point in the barrel harmonics on each shot. If the barrel harmonics vary, so will accuracy.

The AR-15 piston system, working in or on the barrel block the new system requires, adds mass and potentially vibration, and also can potentially bind the barrel as the barrel heats. (Binding depends on how securely the AR-15 piston system is held by the barrel/receiver geometry.) A superb barrel will have few or no stress lines in it. A bad barrel can have many.

The stresses can be from the original steel bar, or be added in the machining or straightening process. As the barrel heats up, the stress lines “unkink” and the barrel points differently. It also changes the harmonics, and thus, potentially, accuracy. (A brief aside: hammer-forged barrels have the stress lines pounded out of them, and cryogenically-treated barrels have the stress lines relaxed.) If the AR-15 piston is a firmly-held object between block/barrel and receiver, it can lever the receiver as the barrel heats up and unkinks.

The extra AR-15 piston parts can hold heat. Also, as the barrel expands as it heats, the piston parts heat up at a different rate, and add another potential binding or pressing on the barrel.

The piston itself can also influence accuracy. When the M1 Garand was the king of the target range, everyone knew that if the op rod got bent, accuracy went all to hell. Bending op rods usually happened when someone used the wrong powder, one outside the burning rate range the Garand would accept, and the rod was over-worked. But once bent, it was “goodbye accuracy” and the situation could be restored only with a new, correctly-dimensioned op rod.

When the M14 became the target king, it did so only after armorers figured out that the barrel could not be free-floated and had to be pre-stressed. The USAMTU match specs call for welding the gas system and front plate together, and using that as a lever to pull the barrel down as it is locked in the stock. The barrel starts out pre-loaded downwards, dampening the harmonics. If the bedding goes, the pre-load changes, and accuracy goes kerflooey. However, no need to replace parts there. “Simply” re-bedding will do. However, every time the action was removed from the bedded stock, the bedding suffered a bit. Match shooters using the M14 became adept at cleaning their rifles without removing them from the stock.

The AR can be free-floated, even with a piston system, but the piston has to neutrally influence the barrel, or your accuracy, zero or both will change as the barrel heats up. With the DI system, not so much – nay, hardly at all, especially with a good barrel in it.

And, on top of all that, the AR-15 piston system brings with it another problem: tilt. (Actually, two, but I’ll detail that in a bit.) When the DI system pressurizes the carrier, it basically pushes the carrier rearward axially. That is, the direction and location of the thrust is on, and in line with, the center of the carrier itself. Enter the AR-15 piston system, which taps or pushes on the carrier up where the gas key used to be. The carrier tries to tilt in the upper and is restrained from doing so only by the buffer tube.

The buffer tube, being made of aluminum, is not at all happy with the steel carrier slamming down and gouging it. Now, the gouging isn’t all that bad, at least not what I’ve seen of it. And not all (even the early ones) AR-15 piston systems tilt or gouge. Me, if I really felt the need to use a piston system, and found that it gouged the buffer tube, I’d perform a simple calculation: will the buffer tube last as long as the barrel? If it did/would, I’d simply view the cost of a replacement buffer tube as part of the cost of a new barrel, and not sweat it.

If the tube wouldn’t, then a barrel replacement becomes a 2X or 3Xbuffer tube cost. At the moment, a plain old USGI-dimension, six-position carbine buffer tube costs $25. A good barrel (there isn’t much point in buying a cheap barrel) starts at about $200,and that is for a steel tube lacking sight, gas block (you’re going to take off the one for your AR-15 piston system, right?), nut and such.

So, as long as it doesn’t cause a functioning problem, replacing the buffer tube is a fraction of the total cost to replace as hot-out barrel.

Oh, and the second problem with an AR-15 piston system? Cost. If you use a replacement kit, you’ll be replacing the existing carrier with a piston-compatible carrier. If you buy a full-up rifle/carbine, you’ll be paying an extra for the design and fabrication costs of the new parts. Either way, your new piston-equipped rifle is going to cost a bit more than a plain old DI-running one.

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.