The AR-15 is without question the most popular rifle platform in the USA today. With dozens of models, styles, and options to choose from -- there is one choice that might be harder than the others.
Direct impingement or piston?
What are the differences, why you should care, and what is right for you is down below!
What Are Direct Impingement and Piston Actions?
How Does Direct Impingement Work?
Classic direct impingement works by a gas tube tapping a small amount of gas off the barrel and directing it back to the bolt to push the bolt backward and cycle the rifle.
This is a very reliable system since basically nothing can go wrong and there are effectively no moving parts other than the bolt.
However, that is not how an AR-15’s “direct impingement” system works.
In the AR-15 gas is tapped from the barrel, directed through a gas block and gas tube back into the rifle and into the gas key of the bolt carrier group. The gas flows down behind the bolt and fills a gas chamber with pressure thus pushing the carrier to the rear forcing the bolt to unlock and the BCG to cycle.
Direct Impingement or Internal Piston?
I’m going to ruffle some feathers here, but I’ll say it: the AR-15 doesn’t use a real D.I. system, it’s an internal piston system. How and why it started to be called D.I., I don’t know. But it isn’t.
A real D.I. system is like that found in the French MAS 40, the Swedish Automatgevär m/42, or the Egyptian Hakim and Rasheed rifles.
In all of these rifles, the gas tube directs gas so that it directly impacts the bolt/bolt carrier to cycle it. Literally, direct impingement.
The AR-15 doesn’t do this. The gas tube directs the gas into the gas key of the BCG and through there into a void behind the bolt. That chamber is sealed with gas rings and allows the gas to expand and push against the bolt and bolt carrier.
This is by every definition a piston.
Still not convinced? Well, in the words of Eugene Stoner in U.S. Patent 2,951,424 for the “Stoner Gas Operated Bolt and Carrier System” the patent reads:
“Gas is routed from a port in the barrel through a gas tube, directly to a chamber inside the bolt carrier. The bolt within the bolt carrier is fitted with piston rings to contain the gas.”
“The gas is funneled inside the bolt carrier wherein the increase in pressure results in the bolt itself acting as a piston, forcing the bolt carrier away from the barrel face.”
Okay, really it doesn’t make a difference if we call it an internal piston or a direct impingement system. Since the common name for the AR-15 gas system is direct impingement, we’ll still call it that.
Even if the AR-15 isn’t a true D.I. system, it’s a close cousin.
How Does a Piston System Work?
Pistons can either be long or short, they work slightly differently but are basically the same. We’ll cover the differences in a moment.
First -- pistons.
A piston gas system taps gas from the barrel much like a D.I. system does, it feeds the gas through a gas block and towards the rear of the rifle (normally).
But that is where the similarity ends.
Right behind the gas block is the piston. The gas builds pressure against the face of the piston and pushes it backward.
The length the piston goes back is either short or long, but either way it goes back and cycles the bolt/bolt carrier.
Once the piston has gone far enough backward it vents the gas through holes, after the gas vents the piston returns forward ready for another shot.
Short Stroke Vs. Long Stroke Pistons
Piston systems come in two flavors, short and long.
Long stroke is the older of the two and is more what people think of when they think of a piston rifle, mostly because the AK-47 and M1 Garand are both long stroke systems.
Long stroke means that the piston and bolt/bolt carrier are all connected and the piston itself travels with the bolt/bolt carrier for all or almost all the length of travel. This, as with anything, comes with some pros and cons.
The upside is that the gas can push on the piston for a longer amount of time and delivers more energy to the system. If you’re shooting in adverse conditions, that’s a good thing. It also means there are fewer parts and fewer things that can go wrong while also being easier to clean.
But the bad news is that more mass moving back and forth means more felt recoil and a slower rate of fire (not really an issue for semi-auto guns). The bolt/bolt carrier and piston being connected also transfer more heat into the bolt and the action of the gun. This generally means that you’ll feel the heat sooner and more of it.
Short stroke is basically the same idea, but the piston isn’t connected to the bolt/bolt carrier. Instead, the piston only moves about an inch or so with the bolt/bolt carrier before stopping. The bolt/bolt carrier keeps moving due to the momentum it gained while the piston was pushing on it.
And yes, it has pros and cons.
Short stroke systems have more parts since the piston is its own piece plus there is an extra spring or two in there so the piston can return home. More parts mean more things that can break. It also means that there is generally less energy in the system and stoppages can occur under bad conditions. If you’re not dunking your rifle in mud, this isn’t likely an issue -- but it technically can be.
However, short stroke has some major upsides also. Less moving mass means less recoil and faster/more accurate follow-up shots. The entire action also tends to weigh a little less since the piston shaft itself can be more lightweight.
If a piston or spring breaks you can replace it with ease and at low cost since it’s not an integral part of the bolt or carrier.
Most AR-15s that use a piston system are short stroke pistons. There are a couple of long stroke systems out there but they’re rare.
What Is Better, Long Stroke or Short Stroke?
I’ve never seen really detailed testing between the two systems, but from what I have seen I don’t think there is a “better” option.
If I was a betting man (and I am) my money would be that long stroke is slightly more reliable and maybe more durable. But I don’t think the difference would be extreme and it would be a negligible gain for the average civilian rifle owner.
Overall, I generally prefer short stroke due to the weight and recoil. Anything that makes me faster and more accurate without compromising durability is a huge positive for me.
Direct Impingement Pros & Cons
The D.I. system is rock solid and dead reliable. The AR-15 was designed for it and through masterful engineering and some iterative design over the past 70+ years, it is one of the most reliable and durable rifles today.
There is very little that can go wrong with the D.I. system. Over tens of thousands of rounds fired the gas block’s gas hole will start to erode, but the barrel will normally burn out before that becomes an issue. Otherwise, this is a foolproof system.
It is lightweight and highly accurate.
Heat is evenly distributed through the system so that no part gets burning hot too fast.
While some like to claim that D.I. systems “are dirty” this is simply wrong. Yes, more gas and carbon will go into the action of the AR-15 than if you use a piston system. But that isn’t a bad thing. The gas pressure helps blow it out and keep crud out of the action, this is especially easy to see if you drop your rifle in mud or something.
Carbon is also a lubricant. Dry carbon powder is literally used as a lubricant in thousands of industrial applications.
While I would never recommend that you routinely run your AR-15 without a touch of lube, the carbon that naturally coats the system goes a long way to keep the action running smoothly with or without additional oil.
Finally, D.I. is just super simple. And much cheaper than pistons. They are made in larger volume and the price is kept down because of it.
Now The Cons…
This is a little debated and kind of depends on who you ask, but there is an argument to be made that the D.I. system doesn’t drain water as fast as pistons do. This matters if you’re a SEAL or like to go swimming while carrying your emotional support rifle, but for the rest of us, this doesn’t matter.
Less controversial but still not entirely agreed on is that D.I. isn’t the most amazing suppressor host. Because the gas gets blown into the BCG and adding a suppressor increases the gas blown back, it can over-gas the system and spray gas into your face/eyes. This is fairly easy to mitigate with an adjustable gas block and a good charging handle, but it happens.
What is entirely true is that D.I. systems are pickier when it comes to barrel length, gas system length, rate of fire, and the ammo they shoot.
Now that isn’t to say that you can’t adjust all of these within reason and have perfect function, but the limits are a bit narrower than a piston system.
D.I. rifles require timing that is determined by the length of the gas tube, the length of the barrel before and after the gas port, and the size of the gas port used. When these are working together, the rifle works wonderfully. But if one of these becomes out of sync, the rifle starts having issues.
Mostly this is seen with short barrels (10ish inches and shorter) or when the rate of fire is very high (full-auto high) or when the gas port erodes (this takes a LOT of rounds fired, normally 10k+).
Piston Pros & Cons
More or less, pistons are just the other side of the coin.
The upsides to pistons are that they are really not picky about what ammo you feed them, underpowered or overpowered (within safe limits) they’ll run the action either way.
Gas is kept away from the shooter so they’re easier to match with a suppressor without having gas spat in your face.
The action of the rifle runs cooler since there isn’t hot gas being blasted through it. Also, the action is fundamentally cleaner.
And if you want just gotta have the emotional support rifle hugging your body on a quick jump in the pool, pistons don’t hold water and are ready to shoot almost immediately after coming out of the water.
It’s Not All Roses…
Pistons are more expensive. Normally by a lot.
The heat while kept out of the action is not gone, instead, it tends to migrate to the handguard roughly where the shooter’s hand is and can get very hot.
Pistons while being heavier overall also move the weight forward making the rifle front heavy and “feel” heavier than they actually are.
What System Is Right for You?
You won’t go wrong with either a direct impingement system or a piston system. Both are entirely viable, and both will last a long, long time. What is right for you just kind of depends.
While D.I. might be the OG system, piston AR-15s have gained a lot of popularity within the military and SF units both in the USA and worldwide.
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