Up until 1964, most major military forces used semi-automatic and fully automatic rifles with piston-driven gas operating systems. These systems divert a small amount of hot powder gas to drive an internal piston, similar to a car engine, to cyle the firearm’s action. Piston systems generally fall into two basic categories: short-stroke (found on the M1 carbine and AK-47) and long-stroke (found on the M1 Garand).

In 1957, Eugene Stoner, working for Armalite, introduced the direct impingement operating system, revolutionizing firearm design. The direct impingement M16 became the longest-serving battle rifle, and its semi-auto cousin, the AR-15, gained immense popularity in the U.S., earning the nickname “America’s rifle.”

What Is Direct Impingement?

Direct impingement bleeds a small amount of hot gas from each round of 5.56mm ammunition fired. But instead of the gas blowing against a piston and an operating rod, it is carried inside a metal tube above the barrel back to the receiver. From there, it is blown into the bolt carrier, cycling the weapon. This system, particularly in M4 carbines or AR pistols, fouls the action with hot gases, making these rifles dirty to shoot. However, the absence of a moving piston and rod ensures fine accuracy due to unaffected barrel harmonics. A DPMS representative once stated that they would not produce a gas-piston AR-15 due to the potential accuracy loss.

AR Systems Head-to-Head

Gas-piston ARs have seen fluctuating demand. 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. Nevertheless, several manufacturers still produce gas piston AR-15s.
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An example of an AR with a gas piston system.Gas Piston AR-15 Advantages

  • Cleaner Operation: Gas is diverted into the piston instead of the action, requiring less cleaning and lubrication of the AR-15.
  • Reliability: Gas piston guns, like the AK-47 and Ruger Mini-14, are known for reliability. The Navy Seal Team 6 uses the gas-piston HK416 as its primary rifle.
  • Durability: Less heat stress on critical components extends the lifespan of parts.

Direct Impingement AR-15 Advantages

  • An example of an AR with a direct impingement system.Weight: Direct-impingement ARs are about 1 pound lighter on average.
  • Accuracy: These are preferred for precision shooting and often used as sniper rifles.
  • Interoperability: ARs with a direct impingement system usually use mil-spec parts, allowing for compatibility between brands, unlike proprietary piston systems. Many aftermarket parts are available for the direct systems, such as low-profile gas blocks and bolt carriers with specialized coatings.
  • Variety: More styles are available, from pistols to full-sized rifles.
  • Cost: Generally less expensive due to 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?

While gas piston systems offer advantages in terms of cleanliness and reliability, the direct impingement AR-15 provides benefits that many shooters, including myself, find indispensable. Eugene Stoner’s design remains unparalleled in its efficiency and effectiveness. For those wanting less maintenance, a gas-piston AR might be suitable, but the direct impingement AR-15’s advantages make it hard to beat. Learn how to choose the right AR-15 for you.