There is no doubt about it; the AR-15 is America’s rifle. At one time, a bit of an oddity, this lightweight and versatile platform has replaced the beloved Winchester or Marlin lever action as the most common rifle in America. There are two operating systems for the AR:
- Direct Impingement aka DI
- Gas Piston
Piston vs Direct Impingement
Each one has its pros and cons. We’ll look at the direct impingement vs. gas piston debate and help you decide which one is right for you.
Direct Impingement (DI)
The first and most common operating system for the AR platform is direct impingement (DI). If you were to remove the handguard or rail from your rifle, although some rail systems allow it to be visible, you would see a thin metal tube above the barrel (see below).
A direct gas mechanism involves a port in the barrel and the action connected by a hollow tube. Gas from the barrel is directed through the tube to cycle the action.–BBC
That tube is the gas tube. When a round is fired, the expanding gasses pass through a hole into this tube. The gas moves rearward and pushes the bolt carrier backward to cycle the action. The bolt head has an extractor that extracts and ejects a spent case as the gas is forced into the bolt carrier, which rebounds against the buffer and action spring. This spring pushes the bolt forward, and the bolt head strips the next round from the magazine. It then loads the round into the barrel’s chamber to fire again.
It is safe to say that the majority of ARs that you have seen are the direct impingement type.
Gas Piston System
Early ARs and M16s had a problem at times which affected reliability. We can see that this had to do with buffer weights and the size of the gas port, gas tube dimensions, etc.
The solution at the time was to take a hint from the AR’s main rival, the AK-47.
In the AK platform, an operating rod serves as a piston-type mechanism. This can be seen on other semi-automatic rifles such as the Armalite AR-18 (or AR-180), M1 Garand, M14, SIG MCX, etc. But it was more than likely the AK’s influence on the piston-driven AR than these.
With piston-driven guns, a moving piston takes the place of a gas tube. Gas from the barrel drives back the piston rearward to cycle the action.–BBC
The mode of operation regarding the bolt and buffer’s function is the same as the direct impingement type we detailed above. The difference here is that gas pressure pushes an operating rod rearward to strike a lug on the top of the bolt, which cycles the action.
Piston vs Direct Impingement: Which is Better?
That depends on what you want out of a rifle. Since the two systems share some similarities and are essentially both gas-driven mechanisms that unlock a bolt to cycle and load a bullet, you may be wondering why some gun owners prefer one system over another. Well, each carries its own sets of benefits and drawbacks that can affect your shooting style and your personal preference. If you’re wondering which approach is best for you, look at some of the following pros and cons of gas piston vs. direct impingement.
Pros and Cons of Direct Impingement
Direct impingement rifles have been around for well over 60 years by this writing. As mentioned previously, they have been improved and reworked for decades. They are the current standard for AR rifles and pistols because of this legacy. Some shooters still regard it as a backward approach and want it to be refined and different.
Looking at this simple, lightweight tube, it is scaled for different purposes. Rifle, carbine, mid-length, and pistol sizes are available. The length of the tube is the distance from the gas block, which holds the tube in place over the gas port to the inside of the upper receiver, where the gas impacts the gas key on the top of the bolt to cycle the action.
The DI system is better because it transmits less felt recoil to the shooter than a piston-driven system because the gas moving through the tube makes for a gentler cycle of the bolt carrier than the piston-driven system.
All that is not to say that the DI is the clear winner. Remember earlier when we recounted the need for a piston gun? Some DI guns can still suffer reliability issues, which typically rears their ugly head on shorter barreled guns such as carbines, SBRs (Short Barreled Rifles), AR pistols, and PDWs (Personal Defense Weapons). It can also be problematic with a machinegun or a full auto simulator such as a binary trigger, forced reset trigger, etc.
Reliability issues have to do with the timing of the gas tube. You will always want the proper amount of gas. Too much gas will wear and tear on your AR, particularly the bolt and carrier. On the other hand, too little gas will not allow your AR to cycle because it short strokes the rifle. This is primarily a condition in cold weather, but it can happen anywhere.
So, there is less dwell time when you go to a shorter system. Dwell time is the amount of time the bullet spends in the barrel after it passes the gas port and just before it exits the bore.
Steps to address this included going through various gas tube lengths. The AR was invented and, some would say, optimized as a military rifle with a 20″ barrel. This means that the standard gas tube length was around 12″ long. However, the military wanted shorter, more compact carbines, and a shorter barrel resulted in a shorter gas tube. So we got the Carbine length which was about 7″ in length. This worked okay on many 16″ barreled guns but became problematic with that length system on 14.5″, 12″, 11″, 10.5″ and shorter barrels. The main problem was over-gassing.
An over-gassed AR is caused by too much high-pressure gas hitting the bolt carrier key because the bullet has cleared the gas port but has not left the barrel yet. In addition to increased wear and tear on your AR’s internals, you will feel the increased gas to the point of burning your face, mainly if shooting with a suppressor. The bolt’s extractor has been known to rip the back end of the fired case off without extracting it in some scenarios.
To address over-gassing, engineers from all parts of the firearms industry came up with solutions such as a pigtail gas tube resembling a Krazy Straw to get a 12″ tube in a spot where a 7″ would go. Other manufacturers built carbines with a dissipator profile, meaning that the longer tube in the shorter barrel pushed the front sight post and gas block almost to the muzzle.
Perhaps the mid-length gas system was the more prudent solution that bridged the gap between the rifle and carbine length tubes. Other improvements included adjusting the size of the gas port in the gas tube or ensuring that the gas key screws are properly staked.
Additionally, the gas port will begin to erode quickly, which will increase the over-gassing issue.
Combine that with the incorrect buffer or action spring weights and weak magazine springs, and you get a perfect storm for people to say your rifle is garbage and unreliable legitimately.
Another downside of the direct gas impingement setup is the amount of soot and fouling in the action accumulates more quickly than in piston designs. Burned powder and lead dust from the fired round is recycled into the action through the gas tube.–BBC
Summary of the Pros & Cons.
Pros:
- Simplicity
- Less felt recoil
- Price
Cons:
- Reliability
- Increased wear and tear on internal parts due to over-gassing.
Pros and Cons of the Gas Piston System
The gas piston method of operation is not as new as most people think. It goes back to the mid-1960s and another of Eugene Stoner’s designs, the AR-18.
In a piston gun, a metal rod pushes against the bolt carrier, driven by a piston located just behind the barrel gas port. This piston design keeps the rifle cleaner and more reliable by not dumping all the gas into the upper receiver. It is still far from perfect, and as a DI gun, there are several pros and cons regarding piston-driven firearms, with their main benefit being reliability.
Because of the action of the piston, these guns are not very finicky when it comes to different ammunition types. Regardless of the ammunition you shoot, it behaves more like an AK-47 and feeds pretty much anything if the external measurements of the cartridge are in spec.
In a piston gun, the operating rod is doing the heavy lifting instead of the gas in the tube, so the system remains cooler and cleaner when compared to a DI gun.
The piston system weighs a bit more than a traditional DI gas tube, but we’re only talking ounces here, and it doesn’t make much difference in the grand scheme of things. It is still worth mentioning.
Split down the middle concerning pistons is the silencer question. Because you have that operating rod, less gas will vent back into your face. However, the gas bleeds upfront will increase the sound signature, unlike a DI gun.
While I’ve never noticed much of a difference in felt recoil while shooting 5.56 NATO, some of the newer, heavier rounds such as 300 Blackout, 450 Bushmaster, 458 SOCOM, and others seem to have a bit harsher recoil when fired in a piston gun. If these are the rounds you prefer, be prepared for it.
The main thing I don’t like about piston guns is that they tend to be less accurate. The rifles are still more accurate than most shooters, but if you’re the kind of person who goes for sub-MOA groups out of your rifle, you may want to rethink this route. When you have a rapidly rotating operating rod slamming rearward against your bolt, it will throw off your barrel harmonics.
Lastly, my other gripe with piston ARs is the loss of potential. I prefer the piston systems of other rifles such as the AK-47, AR-180, BRN-180, FN FAL, SIG MCX, and the Ruger Mini-14 simply because of the traditional buffer and action spring are not used, and a folding stock can be used instead of a fixed or telescopic one.
Summary of the Pros & Cons.
Pros:
- More reliable and stays cleaner and cooler.
- Not finicky with different ammo types.
Cons:
- Increased sound signature
- More felt recoil
- Less accurate
- Loss of potential
Which One Do I Want
As a rule, I present information to help gun owners make correct decisions. What works for me may not work for everyone else, so we have options. One thing about the AR-15 is that it is a highly versatile platform. If you’re curious about the piston system, you can install a piston-driven upper or purchase a conversion kit to try it out. If you don’t like it, tuck it away for a rainy-day project or resell it.
The opposite is true because either system relies on the lower receiver, which is independent of how the upper receiver is configured for operation.
Because this is America, and we have options!
The DI gun has a piston, too, it’s just contained within the BCG (hint: it’s called “a bolt”). It would be more apropos (and more accurate) to call them DI and OpRod, because a DI system has no OpRod and the gas in an OpRod system doesn’t impinge directly on the BCG.