The AR-15 is designed as a combat rifle utilizing a direct impingement as the original technology, devised by Eugene Stoner in the early 60s. Another ingenious solution developed by the brightest firearms designers of that time was the short-stroke gas piston system, applied by Mikhail Kalashnikov to the AK-47.
How a Piston System Works
The need for AR piston-system rifles was not necessary - or to cite the words of late Col. Jeff Cooper - “An ingenious solution to a nonexistent problem.” Still, some manufacturers, striving for perfection released a line of piston-driven receivers that appear to be the ideal blend of AK reliability with AR ergonomics.
With the standard direct impingement system (DI), gas is deflected from the barrel through a tube and back into the upper receiver to perform the cycling. But it can build up a lot of fouling on the bolt carrier group. This residue can result in malfunctions.
On the other hand, a piston system utilizes a piston drive rod instead of a gas tube. The rod acts as an intermediary to exert force on the bolt carrier group to cycle the weapon. Instead of traveling through a tube, the propellant gas simply pushes the and then escapes out of the gas block well forward of the receiver.
Since there is almost no gas running back into the upper receiver, this system keeps the guns extremely clean and cooler than Stoner’s direct impingement system.
Why Choose a Piston System over Direct Impingement?
The AR-15 is an extremely customizable platform. But as with nearly everything with this rifle, the upgrades you make depend a lot on personal preference. And of course, your intended use comes into play, whether it’s competition, hunting or home defense.
Shooters who prefer piston-driven rifles usually state that these arms are cleaner, cooler and more reliable than the direct impingement system traditionally used in the AR-15/M16 platform. They consider them a good solution for a high-volume fire in training classes or for those who shoot competitively in 3-Gun.
We also tend to agree with the line of thought that gas-piston operating systems work best on a carbine or PDW guns with barrels ranging in length from 4.5″ to 12″. This is due to the generally-accepted rule that the longer the gas system (tube) the better the direct impingement gas system will perform.
On the flip side, gas piston technology generates more felt recoil. It also tends to be heavier than impingement rifles. In addition, piston-driven guns are less accurate, especially for follow up shots. Finally, since these systems are less common than DI ones, they usually tend to be more expensive.
Replacing the Upper Receiver with a Piston-Compatible One
While the piston system alters the mechanics and timing of an AR, the geometry of both concepts is the same. The unmatched modularity of the AR allows you to utilize both systems on the same lower receiver by swapping upper receivers out. Because the DI and piston systems operate independently of the lower receiver, you can switch between them on the same lower.
As the AR is the most adaptable and versatile rifle of our time, you can simply alternate the standard upper receiver with a piston-enabled version. By simply changing the complete upper receiver, the shooter can have an AR-15 upper receiver group with its own receiver, barrel, sights, muzzle device, gas system and bolt carrier group. Or you can choose to use a conversion kit to change your direct gas impingement system into a piston one.
Of course, you should note that some of piston upper receivers alone cost as much as a complete direct impingement gun. Besides the cost, another disadvantage is the non-compatibility of the gas piston-equipped AR-15's with .22LR conversion kits.
Piston replacement parts are expensive and hard to obtain. In addition piston systems are not necessarily standardized like direct impingement systems, so they are not interchangeable between manufacturers.
Best Piston Upper Receivers
In this short list, you’ll find some of the best piston upper receivers presenting one of the biggest innovations in the development of the AR-15 uppers.
Adams Arms’ system allows your rifle to run cleaner, cooler, and more reliably due to its patented short-stroke gas piston. Their enhanced piston system involves a small chamber specifically designed to capture the gases.
The company developed its operating rod with a small cup on the end into which the regulator is inserted. It resembles the old “short stroke” piston solution found on the SKS and M1 Carbine.
This mid-length piston system incorporates a Samson Manufacturing Evolution extended free-floating 12″ forearm. This gives you a longer sight radius and a more extended, ergonomic grip.
The Adams re-designed piston upper receiver is forged from 6061 T6 aluminum and features an A4 13-slot Picatinny-cut flattop rail and M4 feed ramps. They provide EVO uppers with their 16"-long standard barrel topped with an A2 flash hider and an adjustable gas block. The gas system can be regulated for use with sound suppressors. It also has a shut-off to allow single shot fire with no cycling if needed.
For all its strong points, serious shooters may not be content with the lack of adjustable settings.
The Adams Arms economic Basic Piston Upper Receiver meets all the requirements of a short-stroke piston system. It helps eliminate hot gas and debris from the action upon firing. That creates a smoother feeding, less violent cycling and reduced felt recoil.
This basic Mid-upper is machined from 6061 T6 aluminum forging and features a 13-slot Picatinny-cut flattop rail and M4 feed ramp cuts.
The Adams Basic piston upper receiver is based over 16" barrel with a mid-length gas system for a smoother recoil impulse and longer sight radius. Along with the 16" Melonite-finished barrel and A2 flash hider, this assembly includes a 1-piece bolt carrier group made of solid steel, piston tube, a steel piston, a charging handle, a delta ring assembly and a Picatinny-rail gas block.
Additional features include an M4-style carbine handguard and a forged A3 flattop configuration with a 13-slot Picatinny rail. The low-profile railed gas block is adjustable for regular fire or for firing with a suppressor The gas-off features enables single-round firing without cycling.
While the gas piston operating system is keeping the gun cooler, the hot gases are directed at the railed gas block. So it’s a good idea only to use a metal front sight mounted here so that it won’t melt atop an overheated gas block.
The Wolf A1 complete AR-15 Upper Receiver is, in fact, based on the design of the R.O.C. T91. But it can be easily paired with any standard AR lower. Although it looks like any other M16/M4 clone, it offers the added reliability of the short-stroke piston system of the Taiwanese T91 military rifle. That model is internally similar to the Armalite AR-18.
Unlike most of the piston upper designs for AR-15, the A1 system is modularized and can be easily removed for cleaning and maintenance
This upper comes complete with a protected front sight tower, a one-piece bolt-carrier group and a charging handle. The package does not include a forward assist button rear sight. In addition, the 16.5-inches cold hammer-forged barrel is topped with an A2-like flash hider. This flash hider has porting further back than normal to act as an effective compensator.
With an authentic charging handle and a handguard, the T91 piston upper fits any AR-15 or M16 lower receiver.
The only drawback we’ve heard about this model has to do with the appearance of this handguard. However, this upper has a utilitarian foregrip that resembles the old American M16A1 clamshell. TIt provides a secure grip and protects your support hand with a heat shield even with heavy fire.
This affordable and proven Wolf Arms piston upper is a good choice for home defense, recreation or hunting. It’s also compatible with select-fire capable lowers, making it the perfect option for tactical applications.
The Alternative: Upgrading With a Kit
In addition to the more common complete AR-15 piston upper receiver, another recent trend is the piston conversion kit.
We all agree that piston operating systems keep the bolt carrier cooler and cleaner, but they also include more moving parts. While it requires no specialized tools or gunsmithing, for mounting this hardware isn't easy for people unfamiliar with the AR`s installation process.
The Best AR-15 Piston Conversion Kits
Let’s have a look now at some of the best piston-conversion kits for the AR-15.
Adams Arms produces their Tactical P-series of solid, reliable piston uppers designed for hunting, competition or duty carry. The Micro Gas Block piston kit is a part of this series. It’s based on the short-stroke free-float piston design preferred by many AR owners.
This updated version weighs 40 percent less than their previous kit. The kit includes a compatible one-piece bolt carrier, installation rod, bolt spring, non-adjustable gas block, bushing with attached spring and upper receiver bushing.
The advanced geometry of the Micro Gas Block expels carbon and gases forward, away from the operator. The installation instructions make converting the AR-15 from direct impingement to gas piston easy. However, the main issue is that it's not attached as it would be with a factory piston.
Another patented design comes from the Superlative Arms LLC. This one from SA offers an adjustable gas piston system for the AR-15 platform which is a relatively simple and effective DIY upgrade.
This kit is available in many configurations such as Pistol, Carbine, Mid and Full Rifle length. It’s also available as a clamp-on or with socket head cap screws. And of course, there are versions for both .750" or .625".
The package included a piston block assembly, a carrier without the bolt, the gas block plug, bushing and an insert tool and the hex keys.
This system will function in rifles chambered in .223/5.56 x 45mm NATO. This model features SA’s proprietary “Bleed Off” gas piston system where the gas flow is regulated by bleeding the gas out of the block instead of restricting the flow like conventionally adjustable blocks.
There are 30 locking gas positions with audible clicks to choose from for many types and brands of round. In other words, you may switch between bullet weights or move from suppressed to unsuppressed fire without any operational problems or causing the system to be under- or over-gassed.
While this upgrade kit appears to handle any bullet weight just fine, there is a noticeable difference in recoil depending on suppressor use. It’s not extreme but it is noticeable.
Just to start on an interesting note: Jason Adams, the founder of Adams Arms also founded Syrac Ordnance. Syrac`s adjustable gas piston system was developed in collaboration with Superlative Arms. So all the products we’re looking at here are closed related.
Generally, all piston systems work the same. But the Syrac`s Gen II series of low-profile fully-adjustable systems offers one of the lightest piston kits on the market. It can also fit under most handguards.
Syrac offers one of the most sophisticated adjustable gas blocks capable of transforming punchy 5.56 AR-15 to a flat and soft-shooting firearm. It’s amazing the difference it can make in the same gun just by reducing gas pressure. This new piston system features a gas adjustment screw that is accessed from the front of the gas block.
The gas block has 24 gas adjustment settings allowing you to fine-tune your rifle to the type of ammo you use. In addition, the adjustable gas block can completely turn the gas off which would cause the rifle to not cycle.
In order to lock in the adjustment, the Syrac Ordnance has developed a locking detent for the screw. That way, the adjustment will never shift accidentally, even under recoil.
There are a couple small complaints about poor instruction and customer service. Also, quality control seems erratic with these.
Over the years more people have become aware of how altering the gas system on an AR-15 can affect overall satisfaction with the gun. Piston conversion kits or dedicated piston upper receivers offer cleaner use of the rifle.
This substitute for the direct impingement system has gained traction in many quarters. Like all changes, it has its pros and cons. But making the change is relatively simple and you can give it a try as part of your enjoyment of your rifle!