Is Your AR-15 Over (Or Under) -Gassed? Here's How to Tell

Does your AR feel a bit too punchy in the shoulder? Are you pulling your bolt after a modest range day, only to find it's caked in carbon fouling? Getting a little too much heat in your face when you run a suppressor? These are all symptoms of the likelihood that your AR-15's over-gassed. There are also plenty of symptoms of an under-gassed AR.

What Does an "Over-Gassed" AR Look Like?

The AR-15 functions by harnessing the gas produced by the ignition of the cartridge in the chamber to cycle the bolt, which cycles back and forth to eject and chamber cartridges, and to reset the hammer and trigger. While most of this gas exits the muzzle, some of it is siphoned off by the gas block via the gas port drilled in the top of the barrel to perform these critical functions.

When too much gas is siphoned and sent back into the upper receiver, the bolt carrier group cycles with excessive force, increasing felt recoil. Excess gas also forces extra heat and carbon fouling into the receivers, which can cause premature wear and tear on components and make for additional cleanup when you break your AR down after shooting.

In extreme cases, this extra gas and fouling can cause reliability issues by "gumming up" the bolt carrier group and upper receiver, which could inhibit the motion of the BCG and cause jams.

Symptoms of excess gas are particularly noticeable when running the gun with a suppressor, especially when paired with supersonic ammunition.

What About an "Under-Gassed" Gun?

An under-gassed AR suffers the exact opposite problem of an over-gassed gun: Not enough gas is being siphoned from the barrel's port and sent back into the bolt carrier group. This is even more problematic, because the lack of gas pressure means the bolt carrier group will fail to fully cycle.

Commonly called a "short-stroking" bolt, this partial cycling of the action often fails to eject spent casings, reset the hammer and trigger, and chamber fresh rounds.

In either case, an AR-15 suffering from too much or too little gas pressure is an unreliable gun.

In either case, picking the right gas system length for your caliber and configuration is the best way to avoid dealing with gas issues.

How to Tell if Your AR's Over- or Under-Gassed

Thankfully, diagnosing either of these problems is incredibly simple: Just watch where your spent casing land on the ground next to you. The amount of gas pressure being sent through the bolt carrier group directly affects the ejection pattern of spent shell casings. Below is a handy chart visualizing different ejection patterns that indicate whether your AR's upper is receiving too much or too little gas:

Ejections Between 12 and 3 o' Clock: Over-Gassed

When the bolt carrier group receives too much gas, it ejects spent casings with extra force. In this case, casings strike the deflector on the side of the upper receiver at excessive speeds, causing them to violently rebound in the opposite direction. When over-gassed, casings typically wind up to forward and to the right of your position, somewhere between 12 and 3 o' clock. You may also notice casings are simply flying through the air at excessive distances, landing between 15 and 20 feet away.

Ejections Between 4:30 and 6 o' Clock: Under-Gassed

Inversely, an under-gassed AR tends to eject casings too far rearward -- usually between the 4:30 and 6 o' clock position. Casings also tend to simply fall flat, landing 2 to 3 feet away from your position.

Ejections between 3 and 4:30: Optimal Gas Flow

Casings ejecting between 3 o' clock and 4:30, and those landing about 8 to 10 feet away, indicate optimal gas flow through the bolt carrier group.

How to Fix An Over-Gassed Gun

Thankfully, fixing an over-gassed AR is easy. Replacing your factory gas block with an adjustable block provides the perfect remedy. An adjustable block allows you to partially restrict the amount of gas flowing through the port of the barrel, forcing it to instead travel out the muzzle.

To tune the AR's gas flow with an adjustable block, repeatedly load a magazine with 3 to 5 rounds, and shoot a few groups downrange. After each firing a group, restrict the block's flow until the bolt fails to lock to the rear on the last round. Once you reach failure to lock the bolt, reverse direction and open the gas block back up one setting.

Verify correct brass ejection with the chart above, and your AR should now be properly adjusted.

How to Fixed an Under-Gassed Gun

Fixing an under-gassed AR is a bit more involved, but still pretty straightforward. In most cases, a gunsmith can simply open up the gas port on the barrel a few thousands of an inch, increasing gas flow to the upper receiver. The end-user fix involves swapping your AR's barrel for one with a shorter gas system length.

Before investing in either of these fixes, first check correct installation and alignment of the gas block with the port in the barrel. During assembly, it is possible to install the block so that it fails to align with the barrel's port. Dimpling your barrel guarantees proper alignment of the gas block, and helps to ensure a tight seal between the block and barrel. Here's how to do it.