Firearm piston systems aren’t new, with the first being patented by William Curtis in 1866. 

In that patent, he also designed the first pistol with a piston system. A multitude of piston designs have been patented since, but the long and short-stroke designs powering semi-automatic guns are the most common today. 

The growth of both piston designs continues in a firearms industry dominated by direct impingement AR format rifles. Obviously the AK formats have survived and thrived all thanks in part to the amazing reliability and simplicity of the AK long-stroke piston design, but many other piston systems have been innovated around a short-stroke design.

With the exception of the long-stroke AK format, short-stroke systems remain the dominant piston system. Some might remember that Ruger first launched its AR platform with a unique short-stroke piston system. The BRN-180 uppers designed by Dean Sylvester have delivered on the legacy of the AR180 short-stroke piston reliability tested into the 10,000-round count. Piston systems can increase overall service life. CZ’s short-stroke Bren 2 has not only drawn huge U.S. attention, it has been battle tested in the Ukraine with a purported 20,000-round service life. 

Similarly the Caracal CAR816 short-stroke piston design with AR15 lower compatibility also notes a 20,000-round service life. The insanely reliable Sig MPX 9mm uses a modified short-stroke piston/tappet system and a short-stroke system in many of their other rifle caliber firearm offerings. Interestingly, the team that originally designed the HK-416 also designed the Sig MPX, Sig 516 and Caracal CAR816, making improvements and advancements over time.

Innovation of the long-stroke piston systems is still happening. IWI’s Galil, Tavor, and x95 have proven themselves with a modified long-stroke piston in real-world global engagements. The PWS Mod 2 long-stroke piston system has been famously noted as the most reliable AR platform ever designed in a premier-tier package.

Regardless of the piston design, piston systems deliver cleaner running reliability typically three times that of direct impingement firearms. Factory-built piston guns have proven their reliability advantage by greatly surpassing the typical failure points of direct impingement guns, but even DIY short-stroke piston kits from Superlative Arms are delivering these same advantages to builders.

Explaining the Differences

The easy customer education point is that with direct impingement guns, the gas pressure directly powers the bolt carrier group and action, but simpler equals hotter and dirtier in this case. With Piston guns, the gas pressure indirectly powers the action with an op-rod. With the op-rod acting as a middle man between the gas pressure and the bolt-carrier, this keeps hot gas at the front of the gun, delivering a cleaner and cooler running gun. Regardless of the short- or long-stroke system, piston gun reliability is primarily due to a cleaner and cooler-running gun design. As with everything, there is a tradeoff and that usually comes in the form of a more expensive and heavier piston gun.

Direct Impingement

Obviously, the vast majority of AR15s and military M16s run on direct impingement gas pressure. It is very reliable and inexpensive but usually has a 1,500- to 3,000-round cleaning cycle to maintain reliability. Gas is bled off from the port on top of the barrel which blows hot gas back through a small gas tube that pressurizes the rings on the bolt. That action drives the carrier back, unlocking and cycling the bolt carrier group. The downside is that heat, unburnt powder, lead particles and debris are blown back into the action with each shot and ultimately build up to the point of impacting reliability. There is also a potential for metal fatigue over time, but the main culprit in reliability is the build-up particulate and debris accompanying the hot gas.

Piston Systems

Most piston systems are either short or long-stroke systems. With gas piston systems, the gas bleed-off process is similar; however the gas bleed-off from the barrel pushes a piston and op-rod assembly attached or floating inside the gas block. The piston is pushed by the gas which moves an attached or unattached op-rod, That in turn pushes on a special bolt carrier and ultimately cycles the bolt. 

The long-stroke piston systems usually have a carrier-attached op-rod with long piston/carrier stroke that rides with the bolt though the entire bolt carrier stroke - i.e. long-stroke. In some systems like the AK, the piston, op-rod and bolt may be a single attached part, but other designs like Galil and X95s have a segmented op-rod. On short-stroke systems, the op-rod floats and is not attached to the carrier. The op-rod provides a quick short push stroke to the carrier - i.e. short-stroke. In tappet systems like the MPX, the push stroke is extremely short.

Advantages & Disadvantages

Direct Impingement - The advantage of the DI system is that it is simple, lower cost, usually lighter, and has less reciprocating and barrel mounted mass. Assembly precision is not as critical with direct impingement systems as piston systems. Direct impingement port alignment, gas-port size, and fore-aft gas tube length can be off by a little and everything is likely going to work just fine. 

Though the accuracy advantages of the DI system is debated, personal testing experience indicated that there is little advantage. The featured Feddersen/SanTan Tactical custom build accuracy was tested swapping between Superlative Arms DI and Piston kits on the same build and provided no variance in accuracy.

The most significant disadvantage of the DI system is the gunk and debris that is blown back into the action with hot gas. With the addition of an adjustable gas block to allow tuning of the gas pressure, recoil can be greatly reduced (typically by about a third), and less gas pressure, heat and debris is blown back into the action. In theory an adjustable gas block can deliver a clearer and longer-running DI gun. With the use of suppressors though, gas pressures rise again and excessive gas is blown back into the shooter’s face if tuning is not made for suppressor use.

Piston - The main advantage is that piston systems do not blow hot gas or debris back into the action even with the addition of a suppressor. If a customer is planning on purchasing a suppressor, a piston system will generally deliver a more pleasant experience. The cleaning interval of piston systems typically exceeds 5,000 or more rounds without a significant impact on reliability. Many shooters have noted 7,000- to 10,000-round cleaning intervals. My experience with previous builds and tests has been that a piston-driven gun seems to run so deceptively clean that I tend to forget completely about routine cleaning.

Precision Fit Requirements - Uniquely, the AK-style long-stroke gas systems generally are less sensitive to the precision fit requirements of the short-stroke piston design. Short-stroke systems must have near perfect alignment and fit between the op-rod and carrier. Without that precision, the bolt face and op-rod will destructively peen each other and the overall system will have functional issues. Once properly set up though, they do not require adjustment.

Cost - Disadvantages of piston guns usually include higher cost due to more parts complexity, and increased weight of the piston system components. Piston systems typically have more mass in the system which means they can soak up a bit more energy during each cycle if well tuned. Without a well engineered and tuned piston system, recoil impulse can feel sharper than direct impingement. Factory tuning the piston ports was a focus on CZ’s Bren 2 and Caracal CAR816 with significant testing of port tuning to reduce recoil. Most gas systems now offer adverse, standard and suppressor multi-position gas blocks. KNS now offers adjustable gas pistons for AK and Galil formats. With suppressor use, piston guns do excel delivering a much cleaner firearm without gassing back into the shooter's face. 

Recoil - Though most shooters would agree the AK-style long-stroke piston systems are usually very over gassed, big leaps have been made by manufacturers with short-stroke piston systems. The gas tuning of the CZ Bren 2 and CAR816 delivers the feel of a competition-tuned rifle with a recoil that feels almost like a rimfire. The CZ Bren 2 is an example of how soft and flat a well-tuned short-stroke piston system can be. The factory gas piston does not have a suppressor setting, however the 30-second upgrade of the KNS adjustable piston does, along with near infinite adjustability and delivers that same soft and flat shooting even with a high back-pressure suppressor. 

Similarly, I built a custom AR featuring a Superlative Arms Piston System, Feddersen barrel, SanTan Tactical receiver set and with all the adjustability of the Superlative Gas Bleed-Off system. Superlative’s piston system is simple for most DIY builders and delivers all the soft and flat shooting reliability expected in a piston firearm with a custom build quality anyone could want.

Direct impingement still leads as the most popular gas system for non-AK format, however the advancements of the next generation of lighter and more durable piston systems certainly cannot go unnoticed.