Semi-automatic rifles have dominated the firearm industry for quite some time, and for good reason. They’re excellent tools for all applications, with there being several rifle models designed and equipped for everything from recreational and competitive shooting to duty and personal/home defense.
No matter which model you look at, they all work by harnessing the energy that’s produced by firing a cartridge, capturing the gas to cycle the action of the rifle. However, once you start looking at the different rifle variants available, you’ll see that they don’t all use the same gas system. The AR-15 uses what’s known as a direct-impingement gas system, while rifles like the SIG MCX and AKM use gas piston systems.
Each gas system achieves the same goal of cycling the action, but they do so in different ways, and they each come with their own advantages. For new enthusiasts, the common question that arises is, “which is best?”.
Direct Impingement vs. Gas Piston: What are They?
Direct Impingement: Overview
So, what exactly is the direct impingement gas system? Direct impingement is a common method of operation for rifles where gas gets siphoned from the barrel and is redirected back to the bolt. This gas directly impinges the bolt (hence the name direct impingement), cycling the action without needing a piston. The concept of DI gas systems can be traced back to the late 19th century with the development of the Rossignol ENT B1, an experimental rifle developed in France in 1896. Later, in 1940, the first production rifle to use a DI gas system, the French MAS-40, would be adopted.
Today, DI gas systems are most commonly associated with rifles like the AR-15 and AR-10. Interestingly though, the AR platform’s DI system isn’t technically a true DI system. In traditional DI systems, the gas siphoned from the barrel is directed back to the bolt face, but on an AR-15, the bolt carrier group effectively functions as a gas piston. Attached to the bolt carrier is a gas key, which houses piston rings which capture the gases to cycle the action. This makes it an expanding gas system rather than a true DI system. Still, the AR’s gas system bears more resemblance to traditional DI systems rather than gas piston ones, so for simplicity’s sake, they’re lumped in under the DI rifle category.
Gas Piston: Overview
Gas piston systems are another widely used gas-operating system for rifles. Much like with DI systems, gas is siphoned from the barrel and directed back to the bolt to cycle the action. However, instead of interacting directly with the bolt like in DI systems, piston systems use the gas to push a piston to cycle the action. Even further, the term “gas-piston” encompasses two distinct gas operating systems: long-stroke and short-stroke systems.
The primary difference between these two gas systems is the physical location of the piston. In long-stroke systems, the piston is attached directly to the bolt carrier, so it moves in tandem with it for the entirety of the cycling process. Short-stroke piston systems have the piston positioned in front of the bolt, where it travels a short distance to push back on either the bolt or operating rod, depending on the rifle. Each system has pros and cons, but we’ll cover them later.
Since the introduction of piston systems, there have been a lot of rifles designed to use them, with some of the most notable examples being the AK-47, FN SCAR, SIG MCX, and the H&K 416. As their popularity has continued to grow, manufacturers have even adapted the AR-15 and AR-10 platform to use such gas systems.
Direct Impingement vs. Gas Piston: Which is best?
At their core, both DI and gas piston systems are useful and versatile, capable of being used to significant effect on many rifle platforms. With that in mind, it’s important to recognize that they each come with their own pros and cons.
Direct Impingement Pros
One of the most significant advantages DI systems offer is their lighter weight. Since DI systems don’t require the use of a piston, they often weigh considerably less than piston operated rifles. Even further, the lack of a piston also reduces the rifle’s reciprocating mass, subsequently making them easier to control. Likewise, this is better for barrel harmonics too, giving DI rifles an edge in terms of accuracy. While piston operated rifles can still be lightweight and maneuverable, DI rifles like the AR-15 excel in this regard.
Additionally, DI rifles tend to be more affordable than piston driven ones. This is due to their simpler design and lower manufacturing cost. AR-15s, for instance, can be found for as low as $600, with more premium options costing upwards of $2,500+, depending on the model. Meanwhile, comparable piston driven platforms can range in price, costing anywhere $1,000 to upwards $3,000+ depending on the model.
Direct Impingement Cons
While effective and efficient, the DI system isn’t without its cons. One of their most prominent cons is carbon fouling. By directing gases directly towards the bolt, excess gas gets vented into the receiver, resulting in higher amounts of carbon fouling on its internals. Over time, carbon will continue to build up, which can cause your rifle to run less efficiently, making it more prone to having malfunctions. Regular maintenance and cleaning are general best practices for all rifles, but it’s arguably more important on DI guns.
This issue becomes even more clear when using a suppressor. Suppressors are a muzzle device designed to reduce the sound signature created by discharging a rifle. They do this by capturing the rapidly expanding gases in a series of baffle chambers, which in turn increases the rifle’s backpressure in the process. This causes excess gas to vent out around the bolt, charging handle, and receiver of DI rifles, inadvertently sending gas back to your face.
If your rifle is over gassed, shooting a DI gun with a suppressor can be quite unpleasant, as these gases cause stinging in both your nose and eyes. However, with proper gas tuning and the right AR-15 gas system length, this can be remedied.
Gas Piston Pros
Gas piston systems are known for their exceptional reliability, particularly in adverse conditions. As mentioned, they direct gas back towards a piston rather than the bolt, resulting in less carbon fouling. This cleaner operation makes piston driven rifles more reliable and less susceptible to malfunctions.
Also, while not true for all piston rifles, most modern options often come with some form of gas adjustment, allowing you to tune to the gas to better suit different applications. For instance, they can house gas tunings for all manner of scenarios, including dedicated suppressed settings to compensate for added gas blowback present when running a suppressor. This is especially beneficial when shooting calibers easily suppressible calibers like .300 Blackout. Our guide, “What is .300 Blackout” goes more in-depth on the cartridge and its capabilities—it’s worth checking out if you’re interested in using it in your next rifle.
Moreover, piston systems tend to run cooler compared to DI systems. A given fact with any type of firearm is that they’re going to get hot eventually. With that in mind, DI systems heat up quickly since gas directly hits the rifle’s bolt and internal components. In piston systems, the piston essentially acts as the intermediary, taking the brunt of the impact of the gases. As such, the heat transfer from the gas to the bolt and internals is minimized, keeping your action cooler, while also prolonging the lifespan of your rifle’s internal components.
Gas Piston Cons
As mentioned above when discussing DI system benefits, piston systems can be heavy. The inclusion of a piston can add several ounces to a rifle’s weight. To compare, your average AR-15 typically weighs around 6.5 pounds unloaded, while a piston driven AR like the Lewis Machine and Tool MARS L weighs roughly 7.4 pounds unloaded—nearly a full pound heavier. Once you factor in a loaded magazine, optics, and accessories, the final weight of your build can be more than you expect, potentially making it less agile and more fatiguing to carry over time.
Another key factor to consider with piston rifles is their accuracy. Although piston driven rifles can be insanely accurate, with rifles like SCAR 20S having an effective range of around 800+ yards, the added reciprocating mass of such systems can negatively affect your barrel harmonics, which can affect your accuracy. This is particularly noticeable on long stroke gas piston systems, where the piston moves in conjunction with the bolt carrier. It’s less of an issue on short stroke piston systems, but compared to DI rifles, they’re both marginally less accurate.
Comparing Short- and Long-Stroke Gas Piston Systems
Going more into detail on the two primary piston systems, it’s important to recognize that neither short nor long-stroke gas systems are inherently better than the other. That said, the operational differences between short- and long-stroke piston systems gives them their own unique advantages.
Short Stroke Piston Pros and Cons
A unique advantage short stroke systems have over long stroke ones is their reduced reciprocating mass. Since the piston isn’t directly attached to the barrel, it moves only a short distance, allowing the lighter bolt carrier to reciprocate the rest of the way, resulting in a softer recoil impulse.
This also gives short-stroke systems an edge in terms of accuracy as well. The reduced reciprocating mass of short-stroke gas systems is better for barrel harmonics, which allows for better accuracy. This isn’t to say that long-stroke gas systems are inaccurate per se, but between the two, short-stroke gas systems tend to win out in this regard. As such, short-stroke piston systems are commonly found across a multitude of platforms, including the AR-15, with select LWRC AR-15 rifles utilizing short-stroke gas piston systems, for example.
Short stroke systems aren’t without their drawbacks. Mechanically, they’re more complex than long stroke systems, incorporating more moving parts into the rifle. The addition of extra components isn’t necessarily a terrible thing, but this does increase the rifle’s potential points of failure. It’s worth noting that modern short-stroke piston systems are exceptionally reliable in general, but their extra moving components give them more potential points of failure compared to long stroke piston systems.
Long Stoke Piston Pros and Cons
As mentioned above, the piston is connected to the bolt carrier in long-stroke systems, thus giving them more reciprocating mass and a more pronounced recoil impulse comparatively. This can affect accuracy, as the extra movement can negatively affect barrel harmonics. Keep in mind that long-stroke gas piston rifles can still be very accurate. Modern AK-47 models, which are generally thought to have an effective range of around 300 yards, can be accurate out to and beyond 450 yards.
However, what long-stroke systems lack in accuracy, they make up for in sheer rugged reliability. Their simpler design results in fewer moving components, giving these rifles fewer points of failure, as well as a more consistent transfer of energy from the piston to the bolt carrier. While no rifle is completely immune from having malfunctions, long-stroke gas piston rifles tend to excel in rough conditions. Over the years, long-stroke piston driven rifles like the AKM, Galil, and Daewoo K2 have garnered significant notoriety due to their reliability alone.
Which is Best?
By now, you’re likely wondering which gas system reigns supreme. Ultimately, it’s impossible to declare one as the superior choice since each one offers their own set of pros and cons. DI gas systems are incredibly common, offering low recoil, as well as a lighter overall weight since they don’t use a piston. However, this comes at the cost of a dirtier and hotter operation since the gas pours directly back to the bolt and into the receiver.
Short-stroke gas pistons have long been respected for having a much cleaner operation compared to DI systems, simultaneously offering more in terms of reliability. Still, they fall short in some regards, as they offer marginally reduced accuracy due to their greater reciprocating mass, while also being heavier due to their inclusion of more parts.
Long-stroke gas piston rifles are renowned for their reliability in extreme conditions, as well as for having a simpler piston design. While reliable, they have much more reciprocating mass compared to short-stroke systems, giving them a more pronounced recoil impulse, and poorer accuracy comparatively.
Conclusion
Between direct-impingement and gas piston operation, it’s impossible to define one as the clear ‘winner’ over the other as both systems can be incredibly useful across a multitude of applications. Ultimately, choosing one is more so a matter of figuring out which one best aligns with your needs and preferences.
A DI rifle like the AR-15 or AR-10 will provide you with enough utility to be useful across basically all applications, while also being more cost-effective compared to piston rifles. You’ll commonly see such rifles being fielded for everything from recreation to more intensive competitive and duty applications.
That said, for those wanting more reliability out of their build, gas piston operated rifles make up for the shortcomings present in DI systems. And since piston platforms like the SIG MCX, FN SCAR, and AKM have become so prevalent, you have plenty of options to choose from, making such rifles a more-than-worthy AR-15 alternative.
At the end of the day, each gas system can be a solid option, but it’s crucial that you choose the right one for your application. Once you narrow down which gas system is the right fit for you, you’ll have no shortage of top-tier rifles to choose from.
