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DI vs. Piston ARs, Tried And True Against The Operators Choice

All the high-speed, low-drag guys seem to be running piston ARs. But are they actually the best? The Direct Impingement (DI) system is sensitive to deviations from the initially designed 20" barrel AR with rifle-length gas system. Piston ARs aim to remedy shortcomings when it comes to specialized ARs. Let's consider the differences, pros, and cons of DI and Piston ARs. We will refer to all types of AR-type rifles as ARs regardless of caliber. 

Clint with his Daniel Defense MK18 SBR, a short-barreled DI AR.

What is Direct Impingement (DI)

DI is a system that takes gas pressure from the fired round to drive the bolt carrier backwards to cycle the action. In traditional DI systems, as seen in the French MAS-49 and Swedish Ljungman rifles, the action is cycled by gas traveling from the barrel to the bolt carrier via a gas tube. When Eugene Stoner created the AR10 in the 1950s, he adapted the DI system to fit his new platform. While both systems, use a gas tube and gas pressure to move the bolt carrier rearward, they differ in how the gas pressure is used. Due to the popularity of the AR, when gun folks are talking about DI they are referring to its use in the AR platform, not in the surplus rifles of yesteryear. 

French Indochina War era MAS 49/56. This rifle represents the original concept of direct impingement.

How A Direct Impingement (DI) AR Works

When DI is implemented in an AR it works as follows: After firing a round, gas flows from the barrel into the gas block, then through the gas tube. Next, the gas exits the gas tube and enters the gas key atop of the Bolt Carrier Group (BGC). This gas proceeds into the bolt carrier via the gas key and expands, pressing on the rear of the bolt. Gas rings on the bolt make a seal between the bolt and the carrier. At this point, the bolt becomes a piston inside the bolt carrier. Expanding gas inside the bolt carrier pushes the carrier rearward which unlocks the bolt and sends the BCG backward under recoil.

The Colt CRM16A1 is a modern reproduction of a Colt M16A1. This rifle represents the original 20in barrel and rifle-length gas system of DI rifles.

Standard DI M16/AR15 BCG. Note the gas key on top of the carrier.

What Are The Advantages of a DI AR

The DI AR has been around for well over 50 years and is a mature and well-vetted design. DI AR's advantages include:

  • DI ARs are less front-heavy due to the use of simple, lightweight gas tubes. The lighter weight front makes it easier to handle. 
  • DI ARs are available with many different gas system and barrel lengths for optimized performance across different calibers.
  • Recoil is mild since gas flows into the bolt carrier and gently moves the BCG rearward. 
  • DI ARs often feature economic pricing.

What Are The Disadvantages of a DI AR

Generally, the more that a DI AR deviates from the original standard 20" barrel and rifle-length gas system, the higher the potential for reliability issues. Potential dependability concerns come into play, especially with short-barreled, suppressed, and or select-fire ARs because:

  • Timing and gas pressure is essential for a DI AR to operate reliably. Shortening the barrel/gas tube lessens the dwell time, this being the amount of time that gas pressure exists in the system. Short-Barreled Rifles (SBRs) exhibit minimal dwell time, which makes case extraction too rapid. 
  • If a case attempts to extract before the bullet leaves the barrel, and pressure has dropped, there can be failures to extract the case. In a worst-case scenario, the case head can be ripped off. If one is lucky, the case sticks in the chamber.
  • Suppressed DI ARs are over-gassed due to increased backpressure. This excess gas leaks from around the charging handle and may irritate the shooter’s face.
  • As the gas port erodes from firing, more gas enters the system. Gas port erosion causes DI ARs to cycle much faster than initially designed. Faster-than-designed cycling speed can lead to diminished reliability, especially in select-fire ARs.

Piston ARs

Did you know that all gas-operated ARs feature some form of a piston? Unlike the DI AR, the Piston AR deviates from the originally conceived design in order to improve reliability in specialized ARs. Typically, Piston ARs utilize one of two stroke operations. Let’s dive into the details below. 

How A Short-Stroke Piston AR Operates

Most piston ARs utilize the short-stroke system that takes design cues from the Armalite AR18. The piston and operating rod (op rod) move a relatively short distance and are not attached to the BCG. Piston ARs feature a gas piston located above the barrel and under the handguard. After firing a round, gas from the burning gunpowder travels down the barrel behind the bullet. This hot gas moves from the barrel to the gas block via a port in the barrel. Gas pressure moves the piston, which pushes the op rod rearward. The op rod then strikes the top of the BCG shoving it back. This motion causes the bolt carrier to move backward, unlocking the bolt and cycling the action. 

Wolf A1 Short-Stroke Piston Upper. Note piston and op rod assembly above the barrel and BCG without a gas key. This upper is considered short-barreled.

How A Long-Stroke Piston AR Works 

Alternatively, other piston ARs use a long-stroke design where the piston, op rod, and BCG are connected. These systems are called long-stroke because the piston, op rod, and BCG travel the entire length of the operating cycle. ARs using long-stroke gas pistons draw inspiration from the gas system in AK type rifles. 

PWS makes ARs that utilize a long-stroke piston system.

What Are The Advantages of a Piston AR

The design lends itself to specialized AR configurations such as:

  • Suppressed ARs are over-gassed since suppressors restrict the exit of gas from the barrel. Piston ARs vent excess gas from the gas block or under the handguard then away from the shooters face. Taking it a step further, many piston ARs also feature adjustable gas blocks where the flow of gas into the system can be adjusted up or down.
  • The short-stroke system limits the amount of gas and fouling that enters the BCG and receivers, which makes for cleaner operation.
  • ARs that are short-barreled suppressed, and/or select-fire fire, are usually more reliable with a piston because the amount of gas that enters the system is more moderated.

What Are The Disadvantages of a Piston AR

Nothing is perfect, and Piston ARs possess a few downsides:

  • The piston and op rod assembly adds extra weight near the front of the rifle. This additional weight can impact handling and the way the rifle swings when transitioning between targets.
  • The op rod striking the top of the BCG during cycling tilts the rear of the bolt carrier down as it transitions into the receiver extension. This phenomenon is known as carrier tilt, and over time, it can cause extra wear inside of the receiver extension. Some Piston AR bolt carriers have a built-up area on the rear of the bolt carrier so that it fits tighter inside the receiver extension, mitigating carrier tilt.
  • Lastly, the handguard area can become quite warm due to the gas block and piston activity.
  • Piston ARs often fall into a premium category, and their prices reflect this. 

Conclusion

The biggest take away is that the further an AR strays from the initially designed rifle length gas system and 20" barrel the less reliable it is likely to be. If you are satisfied with a non-suppressed, semi-automatic AR with a 14.5" or longer barrel, DI should fit your needs. A DI AR in the above configuration is cheaper and lighter than an equivalent Piston AR. If budget is a concern, absolutely go for a DI AR.

If you need a select-fire, short-barreled and or suppressed AR, the benefits of a piston system are undeniable. Remember, however, both systems are incredibly effective and proven at this point. Regardless of what you choose, we live in America where you have the freedom to purchase what you prefer, so buy what makes you happy, just be sure it is from Classic Firearms.

 

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