Some gun enthusiasts swear by direct impingement rifles. Others believe that they’re vastly inferior to rifles using gas piston technology. And then there are the many reasonable folk in the middle who see the merits of both gas operations.
Although the direct impingement system has been around in rifles seen as early as 1940, its popularity exploded when Eugene Stoner used it for his AR-15, which would become one of the most beloved rifles in America. Ironically, the AR-15 doesn’t use the conventional direct impingement system but instead a modified version.
Whether you just want to know more about direct impingement rifles or you’re building an AR-15 and you’re not sure which system to use, here’s everything you need to know about the subject.
Direct Impingement
Your typical gas-operated rifle will have a piston, an operating rod assembly and a separate gas cylinder. A direct impingement rifle will have none of those, cutting down on its weight and its number of moving parts.
With a direct impingement rifle, the barrel has a small hole with a gas tube connected to it. This gas tube runs down the length of the barrel and into the bolt carrier mechanism. Gas travels from the barrel through that tube, where it impinges the bolt carrier mechanism.
The gas gets forced to the rifle’s rear, and at the same time, the casing from the round that was fired is ejected from the rifle port. The spring-loaded action will push the bolt carrier mechanism forward, and it chambers the next round.
Gas Piston Technology
Gas piston technology works much differently. Propellant gases still get bled into the rifle’s barrel, but they then end up in a separate cylinder instead of a tube. There’s a piston in the cylinder, which the gas moves, and the piston then pushes the bolt carrier mechanism to the rear. This ejects the casing from the fired round. After that, the spring-loaded action once again pushes the bolt carrier mechanism forward to chamber the next round.
Choosing Between the Two for an AR-15
Because of how customizable the AR-15 is, you aren’t forced to go with either a direct impingement or a gas piston system. Although direct impingement is much more common and the way the rifle was originally designed, you could choose either one.
To decide which one is the better choice for a build, you’ll need to know their advantages and disadvantages.
Gas Piston Rifles Are Cleaner
One of the most common arguments you’ll hear from proponents of gas piston rifles is that these rifles run much cleaner. After all, the gas is going into a cylinder instead of blasting inside the action of the gun.
This is 100-percent true. Gas going into the action of the rifle will result in carbon buildup on the bolt carrier group, and the bolt carrier group on a gas piston rifle will stay clean much longer.
On the other hand, with gas piston rifles, there are moving parts upfront that could also become dirty due to how it operates. And even though the bolt carrier group gets dirtier with direct impingement rifles, the effect is negligible.
Direct Impingement Rifles Have Greater Accuracy
In my experience, direct impingement rifles have been more accurate performers than gas piston rifles, and other shooters have noticed the same thing.
Something you notice right away when shooting a direct impingement AR-15 is how smooth the entire operation is. The bolt carrier group moves forwards and backwards, and there’s little recoil to manage. The lightweight design makes it easy to aim.
Compare that to a gas piston AR-15. Unlike most direct impingement AR-15 rifles for sale today, a gas piston AR will be a heavier gun because of those extra components, and the motion of the piston leads to snappier recoil. The bolt carrier group isn’t getting linear force, which can throw off your aim a bit.
You can shoot accurately with both types of rifle, but from an accuracy standpoint, direct impingement rifles are the better choice.
Gas Piston Rifles Run Cooler
That gas going into the action of a direct impingement rifle can really heat up your bolt carrier group. That’s not the case with a gas piston rifle.
Is this a big deal? For the most part, no. Sure, you can take the bolt carrier group out of a gas piston rifle immediately after putting 200 rounds through it, but that’s not important enough to affect which option you choose.
Direct Impingement Rifles Are Less Expensive and Have More Part Options
From a price perspective, you’ll typically pay less for a direct impingement AR-15 than a gas piston AR-15. You can find budget models either way, but this is something to consider if price is a concern.
What’s more important is that most AR-15s use direct impingement systems which have interchangeable parts. With gas piston AR-15s, the pistons have proprietary designs and you usually can’t just swap parts in and out. Instead, you need to stick with parts from the manufacturer.
At a minimum, this can be inconvenient if you enjoy customizing your AR-15, and who doesn’t? In the worst-case scenario, the gas piston system could fail and you’ll need to get it repaired with the manufacturer, provided they’re still in business. With all the standard parts available for direct impingement AR-15s, field stripping and repairing your rifle is a much easier task.
Getting the Best of Both Worlds
If you have the money to spare and you want to try both the direct impingement system and the gas piston system with your AR-15, one simple solution would be purchasing two upper receivers, one with each system. You can use the same lower receiver with both of them, which means you could swap the uppers as you please. The other, more expensive option would be to buy two AR-15s.
The Effectiveness of Direct Impingement Rifles
Even though direct impingement rifles have their critics, they tend to perform very well. Gas piston rifles have a couple advantages, namely that they stay cleaner and don’t run as hot, but neither of these are significant.
The dirtier operation of direct impingement rifles hasn’t proven to affect reliability, and many shooters, myself included, have put tens of thousands of rounds through direct impingement AR-15s without any problems. And it’s nice to have your bolt carrier group stay cool to the touch, but that doesn’t offer much in the way of benefits.
With direct impingement rifles, you get a smooth operation that allows you to shoot with excellent accuracy. Your rifle will be much lighter, which you’ll appreciate if you need to lug it around anywhere.
The direct impingement system is simple and effective, making it no surprise that it has become so popular in the AR-15 platform.
