Direct Impingement vs Gas Piston Systems: 2022 Guide

| Last Updated: May 6, 2021

If your day consists of combat,  and you require a rifle for protection, you need the right companion for these stressful situations.

Whether it’s security, military, or anti-poaching units,  you need a reliable weapon with a high rate of fire.

TL;DR: Direct Impingement vs Gas Pistol

Direct Impingement

Gas Piston

Pros

Pros

  • Lighter

  • Less moving parts, less wear on internal parts

  • Cost-effective construction

  • Interchangeability of parts

  • Less moving mass so less sight disruption

  • Versatility of calibers

  • Cleaner and cooler operating system

  •  More positive extraction, chambering, and locking

  • More reliable without the need for regular cleaning

Cons

Cons

  • Weapon internal parts get dirty quicker

  • More friction on the bolts cam system

  • More heat throughout the system burns off lubricant

  • Adjustable gas block required for different operations and lengths
  • More expensive

  • Heavier

  • Parts are not as readily available

  • More reciprocating mass leads to more sight disruption

Best For

Best For

The professional wanting a reliable and affordable rifle platform with access to upgrades and spares.

Military

What is a Direct Impingement and How Does It Work? 

Direct impingement rifle systems are generally cheaper to maintain, lighter in the fore-end, and more accurate than their piston-driven counterparts. Direct-impingements today make up the bulk of ARs on the market, which means there is greater parts interchangeability.

This system works by directing gas through a milled cavity near the barrel's fore-end, the pressure of which is funneled through a thin tube where it impacts the bolt carrier in a gas key situated on top of the bolt carrier. 

The gas, therefore, acts as a piston; the right amount of gas must be funneled down the gas tube to get the desired rate of fire and to avoid malfunctions. This part of the process is where the spent cartridge is extracted and ejected. 

Finally, the recoil spring returns the BCG to the forward position and in doing so strips a new round from the magazine chambers, locking the bolt forward.

Examples of direct impingement rifles include the M16 and AR-15 platforms. Initially, the DI was designed as an infantry weapon, but development has followed a broader target market over the years, namely civilians. It’s now a popular choice for self-defense, as a range option, and in private security industries all over the world.   

Photo credit: gundigest.com

What is a Gas Piston, and How Does It Work? 

Just like direct impingement,  gas is required to move the piston, but that is where the gas's function ends and it is expelled from the rifle. This pressure contact with the piston, in turn, pushes the bolt carrier rearward to address the extraction and ejection of the spent cartridge. The process continues exactly as a DI would.

A firearm using a gas piston system consists of a gas block located anteriorly inside the barrel over the milled section, an over-the-barrel spring, and a guide rod arrangement, all of which are housed and contained in a cylinder that runs the full length of the barrel, maintaining the alignment of these firearm components. 

Unfortunately, this system’s alignment default does not allow for a free-floating barrel to be used.   

Long-stroke Piston

The piston is part of the bolt carrier group and the actions thereof. Once the gas pressure provides momentum to the piston, the piston will follow the entire cycle. Long-stroke is used in weapons such as the AK-47, Tavor, M249 Squad Automatic Weapon, and M1 Garand.

Short-stroke Piston

The piston is separate from the bolt carrier group. It operates in addition to a connecting rod assembly using the energy from the gas. The piston and connecting rod assembly's kinetic energy is transferred into the BCG by a quick push. This energy transfer is enough to continue the full cycle of the rifle. 

Relevant Characteristics between Direct Impingement and Gas Piston

Direct Impingement

Gas Piston

Pros

Pros

  • Lighter

  • Less moving parts, less wear on internal parts

  • Cost-effective construction

  • Interchangeability of parts

  • Less moving mass so less sight        disruption

  • Versatility of calibers

  •   Cleaner and cooler operating system

                    More positive extraction, chambering

         and locking

  •  More reliable without the need for regular cleaning

Cons

Cons

  • Weapon internal parts get dirty quicker

  • More friction on the bolts cam system

  • More heat through the system burns off lubricant

  • Adjustable gas block required for       different operations/lengths

  • More expensive

  • Heavier

  • Parts are not as readily available

  • More reciprocating mass leads to more

  • Sight disruption

Best For

Best For

Professional wanting a reliable and 

affordable rifle platform with access to

parts.

        Military or service personnel who are

         issued their rifle and parts. 

Similarities and Differences

Gas is the primary energy source for both systems. In both, this energy is used to achieve a full cycle, but its effect on each is remarkably different.

DI fouls up a rifle's action very quickly as the gas is ported directly into the bolt carrier.

Gas piston ports the gas out long before it reaches the action of the weapon.

Direct Impingement and Gas Piston Differences 

The main difference you can find between direct-impingement and gas piston is what happens to the gas in the system.

Gas Usage

Direct impingement uses the gas from the fired bullet as the force that moves the bolt carrier group, completing the cycle with the extraction and chambering of the next round. 

It’s a simple and effective method, but here we see that the excess gas is ported down a gas tube directly into the BCG, then expelled from the extraction port, which is very close to the shooter’s face.

Gas piston uses a piston-driven bolt carrier group as a complete unit. So with the piston doing the work, the gas gets ported out of the system earlier, never reaching your face.

Photo credit: defensereview.com

Number of Parts

Direct impingement makes use of a gas block, a gas tube, and connects with the gas key on top of the bolt carrier. With only two working parts this is a pretty simple design for anyone to understand.

Gas piston is a different concept but essentially completes the same function with an adjustable gas block, in which the gas force is used and the excess expelled. The gas piston runs inside a cylinder, which is where it starts getting more complicated with newer systems. 

Some manufacturers using springs work along with the piston to help balance out its effect on the BCG.

Temperatures

A great characteristic of both styles is their ability to function while getting hot.

Direct impingement operates reliably at higher temperatures, but if your rifle does malfunction, you will need to wait a few minutes before fielding striping. The gas piston needs no cooldown period before field-stripping and it runs cooler overall.

Direct Impingement and Gas Piston Similarities

So we have noticed a fair number of differences between direct impingement and gas piston. As for similarities, they can be compared along the lines of temperature and cost. 

Temperature

Direct impingement tends to run very hotly but remains accurate and reliable. Some tests have seen this system rising to 300°F before any malfunction. The dispersion of heat from the gas block through to the bolt carrier is part and parcel of this design.

Gas piston systems run a lot cooler overall but can still maintain accuracy and reliability at similar temperatures to the direct impingement system. 300°F is leaving both these systems on par.

Cost

Cost is a factor when buying any firearm. Consider the AR-15 direct-impingement and the AK-47 gas piston-driven as typical examples; the prices are relatively equal.   

The direct impingement M&P AR-15 Sport II MSRP sells for about $752.00 while the gas piston-driven Century Arms VSKA Synthetic AK-47 retails at $ 749.00. This is as standard as they make them.

Photo credit: thefirearmblog.com

Design Purpose

The most significant similarity we find in these systems is the purpose of the design.

Both direct impingement and gas piston help cycle the rifle after a round is fired. Once the gas reaches a specific section of the barrel, it is forced through a milled area into the gas block.

So here's the difference: the delivery system. DI uses gas-only and piston-driven uses a piston to drive the bolt carrier group. Once this step is complete, it all goes back to the normal function. 

The BCG is forced backward while extracting the spent cartridge; the recoil spring provides the opposing movement, thereby stripping a new round from the magazine and finally chambering the latest round, becoming ready to fire again.

Direct impingement in the M16 platform and gas piston in the AK-47 platform has been around for many years, technologically improving at the same pace. They are almost equally popular with military units around the world.

Pros and Cons of Direct Impingements

Since DI systems have been around for a long time, their old-school approach has some disadvantages. At the same time, it's a tried and tested system.

Benefits

The light weight of a direct impingement system is due to the lack of moving parts. Gas from the fired cartridge propellant directs through a gas block and tube towards the bolt carrier group, creating the inertia that moves the bolt carrier group back and forth.

It is a proven system, finely tuned over the years to be reliable and cost-effective. There is no need for expensive machine parts to drive the bolt carrier. Therefore the only point of wear, without proper maintenance, is in the gas block and tube.

We know the bolt carrier needs to move and without extra parts attached, the moving mass within the system is lower, giving you smoother recoil and less sight disruption.

The AR-15 is one of the most popular direct impingement rifle platforms and works with many calibers and barrel lengths.

Drawbacks

We can see many benefits to this type of rifle, but DI rifles require the gas from the fired cartridge propellant to force/direct the gas tube directly into the firearm's breech, where it becomes fouled more quickly. This is the result of solids and impurities in high-temperature gas condensing and being deposited at the bolt face and primary operating device.

This becomes a maintenance factor and you may even enjoy the servicing of your rifle, but a further drawback to the gas-directed into the breech of the firearm is heat. Friction wears on internal parts like the bolt carrier and extractor. It becomes debatable whether this is a "guaranteed to fail'' system or possibly a means to an end.

You may also note your DI rifle can be more sensitive to varying ammunition. It could be from the fouling, the expansion of internal parts, or the fact that the heat is directed onto the lubricated internal, burning off that valuable layer of protection.

To Summarize

With its long range and accuracy, coupled with the excellent rate of fire, a direct impingement system is a no-brainer for a professional as a fighting rifle. The M16 design has been battle-proven for over 50 years. To date, this design has seen only positive advancement towards the AR-15 in all the various platforms and manufacturer choices. 

One unfortunate drawback to this system is the difficulty in accommodating a suppressor without adjustments or upgrades, but once these necessary tweaks are made, this system becomes barely audible. 

Photo credit: euro-sd.com

Pros and Cons of Gas Pistons

Benefits

A gas piston design can be found on an AK-47, specifically a long-stroke gas piston. This system proves to be a cleaner and cooler operating system as the gas pressure created in the barrel is ported through a gas block, moving the piston and bolt carrier group, thus never entering the breech. 

The excess gas is ported out near the barrel's front through an adjustable gas block, so the system handles high pressure and erosion over time. This improves the ability to suppress your rifle.

Gas piston rifle systems are proven for all kinds of conditions; the AK-47, for example, is known to fire with all sorts of debris inside the operating system. With the piston doing most of the work, the bolt carrier group has more positive extraction, chambering, and locking forward.

This whole system can be disassembled easily without the use of tools, making field stripping the rifle practical if you ever need to inspect it. You won't need to clean your rifle after every range visit; long-stroke gas piston systems remain reliable across many platforms.

Drawbacks

This certainly is a versatile system, but unfortunately, it can be an expensive option. This versatility is used when considering the purpose of your rifle and, depending on your choice, can also limit you to specific manufacturers.

It would then be best for you to research the reliability of a particular system; this can give you a clue as to the manufacturer's intent to continue with its production in the future.

There is no real standard among manufacturers, with many new systems becoming quite complex with the addition of moving parts susceptible to wearing out quicker.

You can't use a free-floating barrel.

To Summarize

The gas piston is a reliable and accurate type of rifle despite the notably harder recoil. Whether you need a range gun or a fighting rifle, most modern-day rifles that are gas piston operated can stand up to some of the worst abuse and conditions.

Your choice of suppressors is vast, but you will hear the rifle cycling much louder on specific makes compared to the direct impingement counterpart.

Bottom Line

For range visits, fouling up the internals of your AR doesn't sound that bad. Still, as a fighting rifle subjected to harsher conditions, some might want a definitive solution to the possibility of malfunctions. With the right upgrades, a DI system can prove valuable due to its versatility, from stealth to regular infantry.   

The AK-47 or similar gas piston rifles use more gas to operate a heavier, more robust action, leaving it less susceptible to damage as a result. The AK is perfect for stealth or infantry using the long-stroke design, but a great example of the short-stroke design is the Negev from IWI, which is ideal for squad support.

DI or gas piston? No longer just the AK or AR debate but rather an operating choice. Both of these have drawbacks and benefits. 

People Also Ask

Direct impingement and gas piston are operating methods of cycling your rifle. These two weapons systems can be similar but one shows exceptional versatility and the other exceptional reliability.

Here are some frequently asked questions for you to consider.

Is AK-47 Direct Impingement?

The AK-47 is a long-stroke gas piston rifle with an intense battle-proven heritage. Renowned for its reliability, this system can run without maintenance far longer than its competitors.

Photo credit: pewpewtactical.com

Is Direct Impingement More Accurate?

Yes, it can be seen as more accurate because it has a smoother action resulting in smoother recoil. The most popular choices of DI run .223 or .556, a slightly smaller caliber than the AK-47, and the system run horizontally; the bolt carrier group runs into the stock of the rifle.

Does a 9mm AR Need a Gas Block?

You don't need a gas system as the AR9 is blowback operated. You will need an AR9 lower, 9mm-specific barrel, a new bolt carrier group, and a 9mm-specific buffer tube. The upper from an AR-15 can be used, but first, it needs to be stripped along with the gas system.



Josh Lewis the managing editor at Gun Mann and when he isn't writing about guns he is more than likely tinkering with them. He also enjoys hunting, fishing and spending time outdoors. As a lifelong gun owner he knows his stuff!