During the 1980s and '90s, gun magazines were obsessed with contests. The obsession didn't lie with giving away prizes but rather "9mm vs. .45 ACP" and "semi-auto vs. revolver" sort of contests. Those types of articles have mostly disappeared, but about ten years ago, a new "versus" appeared — gas piston vs. direct impingement AR-15s.
For your information, semi-autos won, and 9mm has the .45 ACP on the canvas, with the FBI counting to ten.
The acrimonious "mine's better than yours" mud-slinging has been toned down somewhat, and the lines drawn in the sand have blurred, mostly because piston AR people have started making direct impingement guns and vice versa. If you're new to the debate or aren't sure what the differences between the two operating systems are, let's review how they work before delving into their perceived advantages and disadvantages.
The AR-15 is a scaled-down version of the original AR-10, and it has a direct impingement gas system. In the AR, there is a gas port drilled into the barrel. As the fired bullet races down the barrel, it passes the gas port and the expanding gases from the burning gunpowder shoot up the gas port and into the gas tube. AR gas tubes are narrow and usually made of stainless steel and run from the gas block, which is clamped onto the barrel, back toward the upper receiver.
The vented gas shoots back into the upper receiver, where it directly impinges against the gas key atop the bolt carrier. The bolt carrier is then pushed backward, unlocking the bolt. The gas hits the bolt carrier with enough force to send it all the way back in recoil, extracting the spent case. After the bolt carrier has reached its rearmost travel, the recoil spring in the buffer tube shoves the bolt carrier forward, where it picks up the next round in the magazine and chambers it, and you're ready to fire again.
How do gas piston ARs operate differently? Instead of a gas tube that operates the mechanism, there is a piston in its place. The gas comes up through the port in the barrel and hits the front of the piston inside the gas block. The rear of the piston is in contact with the bolt carrier in the receiver, and the gas hitting the front of the piston rod shoves it backward, which moves the carrier to the rear, unlocks the bolt, etc.
Once the piston moves backwards a certain distance, excess gas is vented off from the block, which helps keep the piston/bolt carrier moving at a constant speed no matter the operating pressure of the ammo.
Most, if not all gas-piston ARs are short-stroke designs, which means the piston only moves an inch or less. The piston does not follow the bolt carrier through the entire recoil system. Most designs have a spring around the piston, which returns it to its forward position.
In a gas-piston AR, the gas doesn't directly get the carrier moving as it does in a direct impingement gun. The gas works the piston, which gets the carrier moving. That may not seem like much of a difference, but some people swear by one operating system over another.
Which is better?
If the DI system is the original, you may be wondering what the advantages of a gas piston AR are.
First, gas-piston operating systems are traditionally seen as more reliable. The AK-47, which sets the reliability standard against which all other semi-auto rifles are judged, uses a short-stroke gas piston system. In the early days of the Vietnam War, the AR (in the form of the military's M16) earned a reputation for being unreliable, although this turned out not to be more the fault of an ammunition supplier using the wrong gunpowder than anything else.
Second, because in gas-piston guns the distance the gas has to travel from the gas port to the front of the piston is much shorter than in direct impingement guns, where it has to travel all the way to the receiver, gas-piston guns are less sensitive to pressure variations in ammo. This also means that extremely short-barreled ARs, where the port is close to the muzzle, tend to be more reliable if they are gas-piston designs. direct impingement AR-15s with barrels shorter than 11 inches have historically been unreliable because the distance between the gas port and muzzle is so short — not enough gas can get all the way back to the receiver to cycle the action, but in a piston gun, that gas only has to reach to the head of the piston.
Third, gas piston designs run both cleaner and cooler in the chamber area. Instead of hot, dirty gases being vented into the receiver directly from the bore, the vented port gas only hits the piston. The only dirt and gas that gets into the receiver of a gas piston gun are that which comes out of the chamber as the rifle cycles. The bolt carriers of most gas-piston rifles never get too hot to touch, which is in stark contrast to direct impingement models. Gas piston guns get hot, but that heat spreads from the gas port area, not the chamber.
All of that sounds great, right? So why would anybody want a direct impingement AR-15?
First, while Vietnam-era direct impingement ARs had reliability issues, modern rifles do not. Yes, to paraphrase the naysayers of the design, the AR "excretes where it eats," sending dirty gases into the chamber, but so what? Any properly lubed rifle from a reputable manufacturer should be able to go thousands of rounds between cleanings without jamming. The weakest point in a direct impingement gun (the part most likely to induce a jam) is the magazine, and both types of rifles share the same magazines.
The best example of the inherent reliability of modern gas impingement ARs is the test I performed myself. Back in the 1990s, long before I ever started writing for gun magazines, I bought a Bushmaster CAR-15, which was an affordable alternative to the overpriced Colt. Bushmasters of that era were not considered top of the line or known for their reliability, and machining tolerances and QC were not what you'll find with current day ARs. That said, I shot it regularly at local 3-Gun matches without ever having a problem. To test the gun, I decided to stop cleaning it and see how long it would continue to function without jamming. One year and 1,000 rounds later, I got bored with the test and stopped keeping track, as the darn thing kept chugging along.
Second, gas piston ARs are still relatively new. They did not emerge from an engineer's mind fully formed and have evolved over time. Ever heard of "carrier tilt?" That hard piston shoving back into the bolt carrier was found to cause the carrier to tilt downward in back, causing premature wear and reducing reliability. The solution was to enlarge the outside diameter of the rear of the carrier. Current gas-piston gun designs seem to have worked out all the bugs in the system, but compared to the direct impingement AR-15, which has been in use by the military for 50 years, they're still rather untested.
Third, gas-piston guns have more parts and are therefore both heavier (usually about a pound compared to identical direct impingement guns) and more expensive (usually at least several hundred dollars). Those piston rods have to be made of strong steel, machined to tight tolerances and, if not stainless steel, are usually clad in some corrosion resistant coating. None of that is cheap, and the manufacturer passes those costs onto you, the consumer.
Finally, with everything else being equal, gas-piston ARs seem to have slightly more recoil than identical direct impingement ARs. This was something I suspected before Bill Wilson of Wilson Combat actually voiced it to me during a writer's event about five years ago. He had two Wilson Combat ARs that were identical except for their operating systems — one was a traditional direct impingement gun, the other a gas piston design. While externally identical the gas piston gun, because of the extra parts, weighed a hair more, but it still had harder recoil. I have repeated this test since then, and had the same experience. The difference is very small, but noticeable.
Why do gas piston ARs, everything else being equal, recoil harder? My guess is the nature of the operating system itself. In a direct impingement gun, the expanding gas pushes against the bolt carrier, building pressure until the carrier begins to move, and while that happens very quickly, it is much less abrupt than the smack of the metal piston against the metal carrier in a piston gun. That's why carrier tilt is an issue in piston guns, the harder-than-gas smack. Plus, there is more weight moving back and forth than in a direct impingement gun (the piston adds reciprocating weight), and that translates to increased recoil.
Just like in the 9mm vs. .45 ACP debate, you will have proponents of one type of rifle over another. Traditionally, I have been a fan of direct impingement guns, as I've never seen a need for the gas-piston design. To me, it seems to be trying to solve a problem that doesn't exist — the gas piston guns I've shot haven't proven themselves to be any more reliable than direct impingement guns. Considering I can't remember the last direct impingement AR I shot or tested that was unreliable, I prefer them because they are cheaper, lighter, simpler to maintain and recoil less. Professional 3-Gun shooters get paid to win, and none of them uses a piston AR unless they are sponsored by a company that makes them, which should tell you something.
Several years ago, however, my position on the utility of gas-piston guns changed. If you've ever gone shooting in the summer with a hot sun overhead, you know that, in just a few minutes, a black gun can soak up enough sun to become too hot to touch, even if the ambient temperature is only 80 degrees. Imagine if you were in Iraq or Afghanistan, where it's a hundred and twenty degrees in the shade. And then you get into a gunfight.
Extreme heat (with or without a little blowing sand) can choke a gun to death. With a piston gun, none of the heat of firing gets dumped into the chamber as it does with a direct impingement rifle. Keeping the heat away from the chamber in those extreme conditions, as well as the dirty gases, may just help keep your rifle in the fight.
Currently, the market is flooded with quality ARs at historically low prices, so no matter which type of operating system you prefer, now is the time to buy.