So you’re browsing through a website or store and are bombarded with multiple AR options to choose from.  One of the biggest decisions/concerns, and one we get a lot of questions about is: “What’s the difference between the Direct Impingement (Gas) and Piston operated uppers?” Our last blog post,  How to Change Your AR Handguard,  briefly addressed one of these differences. But we thought it might be beneficial to write a blog that specifically details the differences between the two so that you can make a more educated purchase.

Direct Impingement [gas] Operated AR15

We’ll kick off this shin-dig with the most common AR design, the DI (Direct Impingement) system.

How it works.

Basically, when you fire a bullet it creates a small explosion in the chamber of your AR. The explosion in-turn creates gas which has to go somewhere, and there is only one way to go. So the gas travels down the barrel until it hits the gas block. There, the gas makes a u-turn and heads back towards the chamber through a gas tube. Once the gas makes it back to the receiver it hits the BCG (bolt carrier group) and pushes it back into the buffer tube. The gas is dispelled and the buffer spring pushes the BCG back where it will chamber another bullet or lock back in place if the magazine is empty.

The animation below shows a basic visual representation of what happens in a DI system.

Featured DI (Gas) System AR Upper: SIONICS Weapon Systems Patrol ZERO

Pros and Cons of DI.

Direct Impingement AR Uppers tend to be lighter than the Piston AR Uppers out there. That is because DI systems have less moving parts: gas block, gas tube and that’s it. Fewer parts mean less weight. They also tend to be, but not always, a little less expensive than piston ARs; yet again, probably due to having fewer parts. There are also replacement parts available everywhere so something that broke can easily be addressed and corrected whether in the field or out at the range.

However, DI systems tend to get dirtier. The hot gas that travels through the gas tube carries with it extra powder and residue. These things come to rest in the upper receiver chamber, BCG, and every nook and cranny of the receiver. Now that’s not a terrible thing; all it takes is a brush or a rag and it can be wiped down from time to time. But let’s be honest, not a lot of shooters clean their guns like us other OCD shooters. So if left to build up over a long period of time the residue can cause malfunctions with the AR’s operation.  It’s also super important on both DI and Piston uppers to keep them well lubricated.

Most recreational shooters aren’t going to put the amount of ammo through one of these things that will cause huge build up problems. Still, it’s always good to clean your gun at least once every two – three times you go shoot it (my general rule-of-thumb). OTIS Technology makes a nice portable cleaning kit for ARs.

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Piston Operated AR15

Next up is the Piston System; a.k.a. Gas Piston AR system.

How it works.

So,  the main difference in the Piston system is that instead of a gas tube, it has an actual piston drive rod. When the bullet is fired, the gas travels down the barrel to the gas block. Instead of traveling back to the receiver through a tube, the gas pushes a piston system and then escapes out of the gas block. The piston is then pushed backward into the receiver moving the BCG back.

The animation below shows a basic visual representation of what happens.

The featured Piston System AR Upper: Adams Arms Base Carbine Upper

Here’s slow-mo footage of a Piston System Running

Pros and Cons of Piston System.

Now that you know how it works, lets get down to the brass tacks of it all. Piston System ARs run extremely well. Because there is almost no gas running back into the upper receiver, they tend to run cleaner and cooler than DI guns. If you run a 30rd mag through a DI and a piston AR, the piston AR will come out cleaner. 

On the flip side of things, piston ARs tend to be heavier than DI (Gas) systems. They have more moving parts which not only makes them heavier, it can make them more expensive (Once again, there are expensive DI ARs and some inexpensive Piston ARs; so it all really depends).  When a DI system operates, all the moving is internal; on a piston system, there is external moving that can affect the stability when firing quickly. Parts for the piston systems are harder to come by than they are for the DI systems so make sure and stock up on extras so you will have them for a malfunction at the range or in the field.

There are some great retro or conversion kits out on the market to take your DI gun to a piston gun one such as the Adams Arms Conversion kits. It’s a cool way to test out the piston world without breaking the bank. NOTE: Make sure you follow the installation instructions to the letter so you are not disappointed if it is not set up correctly. There are some great videos out there to help with this process Here:

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So which one should you pick?

As with anything you buy in life, it’s going to come down to personal preference. Some like their Harley’s does that mean it’s better than a Honda. Not necessarily, Harley’s may be better in some places like American made parts and craftsmanship, but Honda’s may be better in price and customizability.

Each AR15 operation system has its perks and its downfalls. It all comes down to your overall goals and mission of the weapon system. If you desire something with a little less maintenance in the cleaning department, the gas piston style would probably be your choice. If you are looking for something that has a vast array of options and parts availability, the DI might be the way to go. Take the time/research to find out what you are wanting out of the weapon and research which would fit your mission best, or you can be like us and have lots of both :).

Hopefully, this Blog has helped you understand the difference between DI (Direct Impingement) and Gas Piston style of AR15 uppers and rifles. Please feel free to send us a comment or question.

We appreciate you reading our blog. If you have any questions please comment below or feel free to email us: Sales@WesternSport.com

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