How to Tell If Your AR-15 is Over-Gassed (& How to Fix It)

We covered how the AR-15's gas tube length affects dwell time, which can also cause the rifle or pistol to be over-gassed or under-gassed. Is your AR short-stroking or, inversely, producing excess felt recoil and throwing spent brass 30 feet across the range's lanes? This is easy to diagnose. Let's review.

What is "Over-/Under-Gassed"?

The AR-15 relies on a steady flow of gas (produced by the ignition of the cartridge in the chamber) traveling from the gas port in the barrel, through the gas block and tube, and into the upper receiver to cycle the bolt carrier group. In other words, the gas system -- and by extension, the bolt -- is responsible for the AR's semiautomatic function, allowing it to feed and eject rounds, re-cock the hammer, and reset the trigger.

Too much (or too little) gas causes these carefully tuned functions to suffer failures. There are myriad issues caused by over- and under-gassing the AR-15, which we'll cover below. So, how can you tell if your rifle or pistol is suffering from either?

AR-15 Ejection Angles

Ejection angles refer to the angles at which spent shell casing exit the ejection port, strike the shell deflector, and ultimately travel through the air before falling to the ground. The angle at which casings leave the upper receiver provide a solid indicator of whether your AR's over- or under-gassed.

The illustration above shows the different angles spent casing may eject from the port on the side of the upper receiver. The areas in red indicate that, should casings consistently fall within either area, the AR is over- or under-gassed. The area shaded green is the ideal range of angles wherein brass should eject with appropriate force.

12:00 to 3:00: Over-Gassed

Casings that eject far forward of, or parallel to, the ejection port indicate the AR-15 has a bolt carrier group receiving too much gas. Casings may also travel excessive an distance while airborne; if you're at a range, casings may land two or three benches to your right, disrupting other shooters.

Symptoms of Over-Gassing

Over-gassed ARs produce excessive felt recoil. You may notice the action appears louder than it should. The buffer and recoil spring may make audible, loud popping and scraping noises, and you may even experience blowback through the ejection port. 

Other symptoms include excessive fouling in the bolt carrier group, upper receiver, and lower receiver after a low round count, and brass deposits being left on the shell deflector from spent casings striking the deflector with excessive force.

How to Fix It

Reducing the effects of over-gassing include installing a heavier buffer to absorb the extra force imparted on the action via the bolt carrier group, installing a heavier bolt carrier group, or using an adjustable gas block to restrict the flow of gas through the gas port into the gas tube.

3:00 to 4:30: Optimal Gas Flow

The AR's gas flow is appropriately tuned when spent casings exit the ejection port between 3:00 and 4:30, or approximately 45 degrees to 67 degrees to the port.

4:30 to 6:00: Under-Gassed

Spent casings that exit the upper receiver far rearward of the ejection port indicate the AR-15 is under-gassed.

Symptoms of Under-Gassing

Under-gassed ARs may short-stroke. In this case, the bolt may only partially travel rearward, failing to properly eject the spent casing, and failing to cycling a new round from the magazine. Stovepipe jams and rounds getting caught on the feed ramps are common symptoms.

The bolt may also fail to rest in the open position on the bolt catch once the last round in the magazine is chambered and ejected. Lastly, if you're a left-handed shooter, you may notice casings striking you in the chest, instead of landing to your immediate right.

How to Fix It

Under-gassed ARs may benefit from a lighter buffer, reducing the resistance the bolt carrier group must overcome to fully travel rearward. A lighter bolt carrier group may also remedy short-stroke issues caused by under-gassing.

An adjustable gas block will likely not fix an under-gassed AR-15. Non-adjustable blocks are already unrestricted; adjustable blocks typically only function to reduce the flow of gas.

The most effective fix requires a gunsmith professionally enlarging the gas port in the barrel to allow for additional gas flow through the block and tube.

How to Swap Your Gas System

Need to swap your barrel and gas system to remedy an over- or under-gassed AR? Follow our guide on How to Build The AR-15 Upper. It details how to install your new barrel, gas block, and gas tube. Just follow the steps in reverse to removing your existing components!

It's also a best practice to dimple your barrel, ensuring your gas block is properly seated atop the gas port. Here's how to do it.

Want to start fresh? A busted gun's always a good excuse to upgrade. We've got quality, tuned AR uppers built in house that are guaranteed to cycle reliably. Shop rifle uppers here! And shop pistol uppers here.