The AR-15 Ejection Pattern Chart, Explained

Does your AR-15 feel like it produces excessive recoil? Or do you frequently wind up with short strokes and jammed casings? Your AR-15 might be over-gassed or under-gassed. You might need to swap your gas system, or install a lighter or heavier buffer. Thankfully, there's an easy way to figure all this out. You just need to observe your AR's ejection pattern.

What is an Ejection Pattern?

That's the angle at which spent shell casings fly out of your AR's upper receiver and bounce off the shell casing deflector. The deflector is that small, pyramid-shaped aluminum protrusion to the immediate left of the ejection port.

Note that if your AR's upper receiver is "slick-sided," meaning it has no deflector, you can't reliably read your rifle's ejection patterns. More on that below.

Why the Ejection Pattern Matters

The AR's ejection pattern changes based on how much speed and force the bolt carrier group is cycling with. That speed and force is determined by two things: Your AR's gas system, and the weight of the buffer inside the buffer tube.

The heavier the buffer and the weaker the gas system, the less speed and force the bolt cycles with. This creates a certain ejection pattern.

The inverse is true, too: A light buffer and stronger gas system increases the speed and force of the BCG's cyclic action, changing the ejectiong pattern.

Reading this pattern is important because it can help you to determine if your AR's gas system is under-gassing or over-gassing the BCG. It can also help determine whether you need to install a lighter or heavier buffer to make your AR cycle properly.

If your AR's suffering from malfunctions but the ejection pattern appears correct, you can typically eliminate a bad buffer and gas system as causes of your malfunctions, and instead focus on other potential sources of failure, like a bad ejector or extractor, or a bad bolt or magazine.

The AR-15 Ejection Pattern Chart

The chart above reflects the AR-15's potential angles of shell casing ejection, relative to the port and deflector. Regardless of caliber or barrel length, the ejection pattern of your AR should generally reflect what's shown above.

In other words, a long-barreled rifle chamber in .308, a standard 16"-barreled rifle chambered in 5.56 NATO, and a short-barreled rifle chambered in 300 Blackout should all have similar ejection patterns when configured correctly. The chart above also reflects potential issues for any of these exampled rifles (and any other AR-type rifles) based on whether its ejection pattern is too shallow or too steep.

How to Read The Chart

When your AR's ejecting spent shell casings at a particular angle illustrated in the chart above, it's either over-gassed, under-gassed, or appropriately gassed.

1:00 to 3:00 Ejections: Too Much Gas

If your shell casings are ejecting far forward and to the right of the barrel, somewhere between the 1 o' clock and 3 o' clock positions, your rifle's over-gassed. This is remedied by installing a heavier buffer, or by reducing dwell time by using a longer gas system relative to your chosen barrel length.

Other symptoms of over-gassing include:

• Excessive felt recoil
• Reduced muzzle velocity
• Excessive carbon fouling in bolt and receivers
• Large brass deposits on shell casing deflector

3:00 to 4:00 Ejections: Ideal Operation (4:00 to 4:30 is Fine, Too)

Shell casings deflecting between just past 3 o' clock and up to the 4 o' clock position are ideal. Casings ejecting between 4:00 and 4:30 indicate your AR is still likely appropriately gassed, though underpowered cartridges might occasionally fail to eject.

4:30 to 6:00 Ejections: Too Little Gas

Shell casings deflecting between 4:30 and 6:00 are not being ejected with enough force. This is likely due to a gas system with too little dwell time, resulting in too little gas making its way through the gas tube and into the bolt carrier group. In other words, your AR's under-gassed. This can be fixed by installing a lighter buffer or, ideally, using a shorter gas system relative to your chosen barrel length.

Other symptoms of under-gassing include:

• BCG short-stroking inside the upper
• Bolt failing to eject spent shell casings
• Bolt failing to feed new rounds from magazine

Changing Gas Systems vs. Changing Buffers

Is your AR's ejection pattern falling outside of the 3 to 4 o' clock range? You're probably wondering which solution is better: Swap the gas system, or install a different buffer? Well, that depends on the underlying issue.

If your AR's slightly under-gassed, simply using a lighter buffer is typically fine. This is an easy fix, and it's affordable. But if your AR's over-gassed, it's better to swap the gas system out, or at least install an adjustable gas block to restrict how much gas gets through the gas port and into the bolt carrier group.

Compensating for an over-gassed AR with a heavier buffer doesn't eliminate the core problem your AR's suffering from: Too much gas is going to the BCG. While a heavier buffer will slow down the BCG and fix your ejection pattern, that extra gas is still imparting too much force on the bolt and its cam pin. Your BCG and other parts in your upper will suffer from extra wear and tear, and potential early failure. 

Not Sure What Gas System Your AR Needs?

We've got your covered. Read our in-depth guide on dwell time. This guide goes into detail about what gas system length is ideal for your AR-15's configuration.