video review
DIRECT GAS IMPINGEMENT
First, let’s address the basics. In the 1950s, Eugene Stoner designed the AR platform to compete in U.S. Army trials for a new military service rifle. Since its adoption, it has quickly become one of the most popular firearm designs across the globe.
The original design incorporated a direct gas impingement system, where gas was redirected from the gas port directly into the chamber against a gas key on the bolt carrier group (BCG). This force pushes the BCG back into the receiver against the buffer tube. As the BCG hits the end of its rearward travel and ejects the spent casing, the buffer tube spring forces the BCG forward again as the next round is retrieved and chambered.
This system was used in the AR-10 and AR-15, which to me means that any deviation from that operating system disqualifies the rifle from being an AR-15.
PISTON-DRIVEN
On the other side of the operating system debate is the gas piston upper. There are basically two major gas piston designs: short stroke and long stroke. Both designs opt to force gas onto a piston head, rather than redirecting the gas directly into the chamber.
LONG STROKE PISTON
The long stroke gas piston is common on many firearm designs, most notably the AK-47. While not typically found in AR-pattern rifles, the concept is important to comprehend.
The long stroke piston features a gas port that bleeds off gas once the bullet passes by. The gas pushes against the piston head that is sitting in a tube above the gas port. The piston head is connected to a long rod that runs the length of the gas tube and is connected to the bolt carrier group. As the gas forces the bolt back, the bolt carrier group ejects the spent casing and the recoil spring brings the bolt back into place, picking up the next round as it moves forward.
SHORT STROKE PISTON
The short stroke gas system is the most common piston design in AR-patterned rifles. This design became prominent with firearms like the M1 Carbine and SKS.
The short stroke system also has a piston head that sits directly above the gas port; however, the piston is not connected to the bolt carrier group. The short piston is connected to a spring and rod that runs the length of the barrel, and a rod that sits above the chamber of the upper.
As the gas pushes the piston, the rod is forced to tap a key on the bolt carrier group, creating enough momentum to force the bolt carrier group back far enough to eject the spent casing and load the next round as the recoil spring returns it to the chamber.
COMPARE & CONTRAST
When comparing direct impingement to the piston system, it is important to determine the pros and cons of each. As always, there is a certain level of subjectivity to this debate, but it is important to list what each system does well in contrast to how it could be improved.
I will evaluate each system by three main categories: recoil, reliability, and suppression.
RECOIL
One of the biggest advantages of the 5.56 cartridge is its soft-shooting nature. The small, intermediate round should be easy to control and should make shooting control pairs and drills an effortless task. Direct gas impingement rifles are usually extremely soft shooting. It is important to note that there are several different gas system lengths, which can have an impact on felt recoil.
Rifle and mid-length gas systems are typically very soft-shooting, while carbine-length gas systems can be over-gassed, forcing excessive amounts of redirected force into the chamber. There is a balance to be struck between barrel length and gas system.
On a rifle like the original M16 with a 20-inch barrel and rifle-length gas system, there is virtually no felt recoil. In comparison, a 16-inch-barreled AR with a carbine-length gas system can be a different story, as the system is over-gassed, and recoil is much more noticeable.
Nevertheless, direct impingement tends to have less mass movement and a softer recoil impulse. On the flip side, the gas piston can have a more pronounced recoil impulse, as there is more mass moving to the rear of the rifle. I have shot several piston-driven 5.56 rifles and I tend to prefer direct gas impingement.
Now, I am not saying I can’t handle the recoil, but if you’re spending long days shooting drills and trying for quick and accurate shooting sequences, you notice the change in recoil. To me, 5.56 should be so soft that it is barely noticeable. In my experience, on most piston-driven 5.56 rifles, the recoil causes the sights to jump around more than a direct impingement design. I give the advantage to direct gas impingement because it is easier to manage recoil.
RELIABILITY
The AR often finds itself in the AR-15 vs AK-47 debate, and many people claim that the AK is vastly more reliable than the AR. So, let’s address how that relates to the gas system.
Firstly, direct gas impingement ARs are typically very reliable. I have shot a couple thousand rounds between cleanings on several of my ARs with no issues.
The unreliable reputation is often fueled by the nature of the direct gas impingement system. Directly venting gas into the chamber of a rifle tends to dirty up the system with carbon and debris. As the round count builds up, the carbon tends to thicken in the chamber and that can lead to reliability issues quicker than a piston system.
Inversely, the gas in a piston system is vented closer to the gas port, keeping the chamber clear from excess debris. Typically, piston systems can go longer between cleanings, as there is less buildup on the bolt carrier group.
In my experience, both systems are reliable, although it is true that you might have to keep up with cleaning and maintenance more closely in a direct gas impingement system. Therefore, the piston system will score a point in this competition.
SUPPRESSION
The last thing to compare is how the systems work during suppression. Firing suppressed is becoming more and more common. Direct gas impingement has one major pro and one major con with suppression, while the piston system has the inverted pro and con.
On the pro side for direct impingement, the system tends to be a tad quieter when shooting suppressed. This is due to the gas being well contained within the system. On the other hand, the piston system bleeds excess gas at the port, causing “piston pop.” I personally do not think it is excessive, but there is a slightly louder bang in the piston system.
On the con side, the direct redirection of gas into the chamber tends to force gas into the face of the shooter, causing a tear gas effect. If you have ever shot a suppressed AR-15, you know the struggle of shooting through the tears. The suppressor also traps more gas, causing the system to run dirtier. This is especially true of direct gas impingement, as the chamber will dirty up much faster.
The same piston pop effect that I listed as a con from earlier actually helps lessen this effect in the piston system. As the system bleeds the excess gas at the port, there is less gas being forced into the shooter’s face. Subsequently, the cleaner nature of the piston system allows for debris to build up at a much slower rate than that of its counterpart.
So, suppressed shooting is sort of a tie for me, as both systems have their ups and downs. I would tend to lean more toward the piston system, simply for the comfort of my eyes while shooting.
ON THE RANGE
To get a proper feel for both operating systems, I did my best to create an apples-to-apples comparison. I took out a Caracal 814 A2 direct gas impingement rifle and compared it to a Caracal 816 A2 piston-driven AR. Both are from the same manufacturer, and both are chambered for 5.56 with 16-inch barrels.
Over a short few hundred rounds, the main range comparison to make is the recoil impulse. Without shooting either system prior, I took each rifle and ran 10 rounds through both systems back-to-back to directly compare the recoil impulse. I was surprised at the results, as they felt almost exactly the same.
I give the benefit of the doubt to the direct impingement rifle, as it has a carbine-length gas system in its 16-inch barrel, causing it to hop around a bit more than I’d like. That said, the piston-driven Caracal was one of the softer-shooting piston rifles I have shot, and I was pleased with the results. Both rifles were a tad jumpy as I ran some drills and control pairs, but both were easy to control. Like I said before, it’s not that the recoil is uncontrollable, it’s just that it could be easier to control.
Each rifle shot several hundred rounds, and the end results were kind of surprising. Aside from the recoil being basically a tie, the piston-driven AR had a couple of hiccups. On three or four occasions, the bolt only moved far enough back to eject the spent casing, but not back far enough to pick up the next round. So, in this case, reliability goes to the direct gas impingement rifle.
SUMMARY
If I had to pick one, I would stick to the direct gas impingement design. To me, Eugene Stoner’s design is so easy to control, and I am a big believer in keeping 5.56 as easily controlled as possible. For that reason, I have normally strayed away from piston-driven 5.56 systems, simply because of the recoil impulse.
As far as reliability, in my extensive experience with different models and manufacturers, both systems are reliable, especially when properly maintained.
Overall, the debate is driven largely by opinion. You may have a different experience that leads you to a different choice. Regardless of where you fall, training is what makes all the difference in the world. You can have the most expensive, reliable, finely tuned AR on the market, but if you don’t hit the range, it won’t matter.
