Up until 1964, all the major military forces that fielded semi-automatic and fully automatic small arms issued some variant of rifle with a piston-driven gas operating system. These systems siphon off a small amount of hot powder gas to drive an internal piston, similar to the piston in a car engine, and this cycles the action of the firearm. They generally were divided into two basic categories: short-stroke piston systems (found on the M1 carbine and AK-47) and long-stroke piston systems (found on the M1 Garand).
Many favor gas-piston rifles for their reliability. But while working for the Armalite division of Fairchild Industries in 1957, Eugene Stoner changed everything via the development of his direct impingement operating system. The direct impingement full-auto M16 has become our longest-serving battle rifle. Its semi-auto cousin — the AR-15 — in the same time period has become “America’s rifle” due to its nationwide popularity.
What Is Direct Impingement?
Direct impingement bleeds a small amount of hot-powder gas from each round of 5.56mm ammunition fired. But instead of that gas blowing against a piston and an operating rod, the gas is carried inside a metal tube above the barrel back to the receiver. From there, it is blown into the bolt carrier, thus cycling the weapon. The introduction of the hot-powder gases (especially in M4 carbines or AR pistols) fouls the action, making these rifles dirty to shoot, though lightweight and capable of fine accuracy. Firing the gun doesn’t interfere with the barrel harmonics due to no moving piston and rod. This leads to the fine accuracy. A representative of the DPMS rifle company told me 15 years ago that DPMS would not introduce a gas-piston AR-15 due to the degradation in accuracy that comes with gas-piston rifles.
AR Systems Head-to-Head
Gas-piston ARs don’t seem to be in as much demand as they once were. Some, like the fine Huldra MkIV Piston AR, are now defunct (along with the company). Other companies like Ruger, who entered the AR-15 field with the gas piston SR556 in 2009, eventually dropped the piston guns from their lineups. However, there are still a number of gas piston AR-15 manufacturers out there.
Gas Piston AR-15 Advantages
- A gas piston operating system runs much cleaner since gas is diverted into the piston instead of the action. That means it needs less attention to cleaning and lubrication.
- Gas-piston guns run more reliably, like an AK-47, Ruger Mini-14 or M1 carbine. I’ve never seen one have jamming problems during training and qualification courses. The Navy Seal Team 6 uses the gas-piston HK416 as its primary battle rifle.
- The parts also have an increased lifespan due to less heat stress being directed against critical components of the action.
Direct Impingement AR-15 Advantages
- Direct-impingement ARs average about 1 pound less.
- These tend to be more accurate and are often preferred as sniper rifles.
- ARs with a direct impingement system usually use mil-spec parts for interoperability between brands. There is no mil-spec for piston-driven ARs since piston systems are proprietary. Myriad of aftermarket parts are available for the direct systems, such as low-profile gas blocks and bolt carriers with specialized coatings.
- There are many more styles of direct impingement ARs available — from pistols to full-sized rifles.
- Direct-impingement guns tend to be less expensive than piston guns because there are fewer moving parts. The only part that moves is the bolt carrier. In a gas piston AR, the piston, operating rod and its linkage are all in motion during the firing cycle.
Which Operating System Is Better?
In the end, I like gas piston operating systems … I just don’t like them as much on ARs. I love my M1 carbine and AK-47, as well as my Garand. Frankly, if I wanted a new gas-piston operated .223/556mm carbine, I would likely purchase a Ruger Mini-14 Ranch Rifle.
The direct impingement AR-15 has too many advantages that I don’t want to give up, even if a couple of those advantages are quirky. Eugene Stoner was brilliant, and he got it right the first time. But if you must have an AR and want to do less maintenance and cleaning, take a look at a gas-piston AR. It may be a gun for you.