Piston ARs

The big dust chamber test pitting the Army’s M4 carbine against three piston-powered challengers is over and the results are more than a little surprising. Even though many expected the M4′s direct-gas system to be outperformed by the cleaner piston systems in dusty conditions, the huge gap between the M4 and all three alternatives is a bit shocking. See Murdoc Online for more details.

Here is a column I wrote for the May/June 2007 issue of Shooting Sports Retailer magazine. My editor has recently given me permission to repost my magazine columns online and I guess I’ll start with this one.

The Next Wave of ARs

Sneaking in under the radar while everyone argues uselessly over what size round the military should use is a change nearly as significant as switching the Army over to new ammunition. It’s the piston-driven assault rifle, and it appears that nearly every AR manufacturer is betting that it’s going to be the Next Big Thing.

When Eugene Stoner’s AR-15, which became the military’s M16, was introduced in the 1960s, the futuristic appearance wasn’t the only thing radical about the weapon. Its action made use of a direct-gas system that was designed to cut down on weight and increase reliability by reducing the number of moving parts that could break. However, the ammunition contract didn’t call for the type of powder specified by Stoner, and the stories of jammed M16s in the jungles of Vietnam are tragically legendary.

Though the system has been tweaked over the years and meticulous cleaning regimens enforced by drill sergeants and squad leaders have cut down on the reliability problems, the M16 and its carbine version the M4 are still notoriously tough to maintain, particularly when compared to the weapon that’s generally employed by America’s enemies, the Russian-designed AK-47.

The AK-47, originally dreamed up by Mikhail Kalashnikov in the last days of World War II, has two features that make it so resistant to stoppages in the heat of battle: loose manufacturing tolerances and a piston. Though professional US troops will not be adopting a loosely machined (and inaccurate) rifle like the AK, the short-stroke piston system, used to cycle the action, could make a noticeable difference and won’t require a total redesign.

The direct-gas system takes some of the propellant gas from the barrel and feeds it back down into the upper receiver via a tube. The gas enters the receiver and the pressure pushes the bolt group back, ejecting the casing from the previous round and chambering the next as the bolt springs back closed. It’s a simple and lightweight system, but all that hot gas doesn’t keep the weapon clean or reliable. First, the heat and residue from the gas attracts dust and sand into the receiver, which can easily foul up the works. Second, the heat is hard on the mechanism. This can cause short-term problems due to tightly machined parts no longer fitting together quite right and to longer-term problems due to parts wearing out too quickly.
A piston system, on the other hand, utilizes a metal rod instead of a gas tube. The gas pushes against this rod (the piston) and the piston pushes back the bolt carrier. This not only keeps the fouling gas out of the receiver, it keeps the heat out as well.

At the SHOT Show in Orlando this past January I witnessed a demonstration by LWRC (lwrifles.com) in which the rep fired six consecutive 30-round magazines on full-auto one right after the other. That’s 180 rounds through the weapon in less than one minute. He then pulled the bolt out and handed it to me. It was warm to the touch but not hot. That’s impressive.

In addition to FN-USA’s SCAR (Special Forces Combat Assault Rifle) which was mentioned in last month’s SHOT Show coverage, another piston AR is entering service with Special Forces. The shadowy Delta Force has selected Heckler-Koch’s HK416 to arm many of its elite operators. Unlike the SCAR, which is a totally new design, the HK416 is outwardly almost indistinguishable from a standard M4 carbine. This means that years of training and experience are not going out the window when a new weapon arrives, and it means that existing accessories (and even many of the current weapon components) are interchangeable between new rifles and old.

Systems like the HK416 have another advantage. They are available as an upper receiver only, and the receivers can easily be dropped onto existing weapons. If you’re familiar with ARs, you know that two pins hold the upper receiver to the lower receiver, and that removing the upper takes only a few seconds. That’s how long it takes to drop a new piston-powered upper receiver onto an existing weapon, and a Special Forces battalion stationed on Okinawa is doing just that as you read this.

Besides the SCAR (available in a law enforcement model this fall and in a black civilian model early next year) and the HK 416 (available now as both an upper receiver only or a complete weapon), there are additional models of piston-powered ARs to keep an eye out for. These include the aforementioned LWRC SRT, the POF-USA P-415, the DSA Gas Trap Carbine, and the upcoming Magpul Masada. Colt even has two models potentially available, the LE1020 and the M5. Also, don’t forget the Armalite AR-180B (mentioned in January’s Battleground) an oldie but a goodie that’s always used a piston instead of a gas tube. These rifles will probably be gaining popularity as piston-powered ARs enter mainstream use with the US military and with private security contractors and law enforcement personnel. And remember that many customers who already own an AR might be interested in just picking up an upper receiver to use on their current weapon.

As the tactical market continues to heat up and military decisions continue to influence civilian buying, piston-driven ARs could quickly become the next hot ticket item. Don’t get caught cleaning your gas tube when you should be paying attention.

–Republished from the May/June 2007 issue of Shooting Sports Retailer magazine. Do not use without permission.