The AR-15 — or M-16 in military parlance — has been a military and law enforcement arm for nearly fifty years and its popularity continues unabated despite the design’s age. Many changes have been introduced to correct real or perceived deficiencies and increase its adaptability and usefulness. Some of these changes have been beneficial and some — perhaps too many — have been nothing more than marketing hype.
One of the most-discussed changes has been the switch from the original gas tube system — known as direct impingement — to that of an external piston to cycle the action.
Here we explore some of the important elements to the piston-driven AR system — along with their associated benefits — in comparison with the traditional, direct impingement systems. In the companion column, I review several piston-driven AR systems from Barrett, Primary Weapons Systems, Lewis Machine and Tool, Rock River Arms, Ruger, and Sig Sauer.
The Mechanics
In a traditional AR system, gas is vented through a small port at the rifle’s front sight. From there, it travels down a stainless steel tube back into the upper receiver. This gas enters the bolt carrier and forces the carrier to travel to the rear and cycle the bolt. This is a very elegant and simple system, but is known to create a rather maintenance-intensive rifle. Switching to a piston drive replaces the hollow gas tube with a solid rod that acts directly against the rifle’s bolt carrier.
Many makers are currently producing piston driven AR’s but the idea of replacing the gas tube with a solid push rod is not a new idea. Colt experimented with gas pistons in the late 1960s and Rhino International sold a conversion kit in the mid 1980’s. Taiwan has fielded a gas piston AR variant for nearly thirty years with her T65, T86 and T91 series of rifles.
The Controversy
There are some “controversial” advantages to the piston concept. The first is reduced cleaning. Piston driven rifles do not vent propellant gasses directly into the upper receiver, eliminating one of the ongoing complaints about the direct impingement system.
Along these lines, piston guns generally run cleaner with a noise suppressor attached to the barrel. Suppressors greatly increase internal fouling and this, coupled with a direct impingement system, can lead to dramatically increased carbon deposits. With a piston drive, the fouling that would otherwise accumulate inside the upper receiver and bolt carrier is directed outside near the front sight block. While piston guns with suppressors attached still get very dirty, most of the crud collects in the barrel extension and is rather localized to this area.
Most of the current piston guns on the market have a specific setting on their gas blocks that allow for a slightly reduced gas port to reduce operating forces during suppressed use. The change in operating method also improves reliability with shorter barrels. Most AR shooters will agree that chopping a conventional AR barrel shorter than ten or eleven inches can lead to a very finicky rifle.
It seems that once the gas port and gas tube get closer and closer to the muzzle, gas pressure can bleed off faster than a gas tube can be pressurized. This can lead to failures to cycle fully. Piston guns are less sensitive in this regard because the propellant gas starts moving the piston as soon as there’s sufficient pressure at the gas port to overcome the piston’s inertia.
Also, since the hot propellant gasses are not blown directly into the rifle’s bolt carrier, piston guns run cooler than their direct impingement cousins. This reduction in operating temperature can help keep vital lubrication from burning off in extended firing sessions.
Even with these advantages, many proclaim Eugene Stoner’s direct impingement gas system to be completely reliable in a maintained and oiled AR. These shooters believe the additional weight and uncommon parts far outweigh any benefit a piston drive might offer.
The Price
The switch from a direct impingement can come at a price. Piston guns, due to the nature of the relatively heavy piston rod and beefy gas block, weigh more than their counterparts. Another unique problem can be what’s called carrier tilt.
The impact of the gas piston at the top of the bolt carrier causes some off-axis force, causing the bolt carrier to rock inside the upper receiver. This rocking motion, if left unchecked, can lead to increased wear and eventual parts failure.
Piston guns don’t, as of yet, consist of any standardized parts which each individual maker producing parts unique to their individual systems.
The Tests
In the companion column to this overview, I review rifles from Barrett, Primary Weapons Systems, Lewis Machine and Tool, Rock River Arms, Ruger, and Sig Sauer.
All rifles were zeroed and fired for accuracy at 100 yards using full metal jacket and boat tail hollow point ammunition and Leupold’s new 1.25-4x20mm Tactical VX-R scope. Once zeroed, each rifle was fired was fired a minimum of 250 rounds to test overall cleanliness. All rifles except the Ruger SR-556CLA, Sig Sauer 716 and Rock River PDS were fired with a Gemtech HALO suppressor attached to see how the use of a noise suppressor affected cleanliness and reliability.
My intent is to set a starting point for those who are interested in buying a piston and experimenting for themselves. Hopefully the potential buyer will find this information helpful in making a more informed purchase decision.
Final Thoughts
Is a piston AR the answer for those looking for a reliable rifle that requires minimal maintenance? Is there one particular design that’s more durable or just plain better than the others?
My answer is a solid, “Maybe.”
Piston guns do run cleaner than their gas tube cousins and should, therefore, require less maintenance.
But, during the course writing this series of reviews, I exchanged emails with retired US Army veteran and former special operations soldier Patrick McNamara. Mac told me that he’s seen the same malfunctions — double feeds, bolt overrides, etc. — on piston guns and thinks the ultimate solution is better education and understanding of AR maintenance rather than a change in gas systems.
While a good regimen of proper maintenance will no doubt help the end user achieve the most reliable firearm possible, the current piston systems will continue to evolve and are here to stay. Evidence of this can be seen with the USMC’s recent adoption of the M27 IAR. The M27 is based on the Heckler and Koch 416 and is being used as an automatic rifle to augment the infantry squad’s firepower.
Now, let’s get to the good stuff, and test some of these systems! Click here for my observations on the piston-driven AR systems from Barrett, Primary Weapons Systems, Lewis Machine and Tool, Rock River Arms, Ruger, and Sig Sauer.
