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Carbine, Mid, or Rifle? A Beginner’s Guide to AR15 Gas Systems

Are you planning on buying or building an AR-15? Or changing out parts such as barrels, gas blocks, or handguards? If so, it’s important to understand how the gas system on an AR-15 works, why it’s so important, and how the length of your gas system can affect the performance of your rifle.

To make it simple, if you have the wrong length of gas system for your barrel, it can cause your gun to not cycle properly, or wear itself out too quickly.

So What Exactly is a Gas System?

Let’s start by asking Wikipedia:
“Gas-operation is a system of operation used to provide energy to operate autoloading firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and chamber a new cartridge.”

So on an AR-15 specifically, there is a small “gas port” (small hole) in the barrel that vents gas with every shot. This gas travels through the gas block and gas tube into the receiver, where it powers the bolt carrier group and auto-cycles the next round. This is known as a “direct impingement” gas system.

The below picture from Wikipedia demonstrates how the gas travels:

Why Does the Length of the Gas System Matter?

The length of the gas system (length from the receiver to the gas port) should increase as the barrel length increases. The reason has to do with “dwell time” – the length of time that the bullet is in the barrel after the shot is fired.

On a longer barrel, the dwell time will be longer, because the bullet has to travel a longer distance. Specifically, there is a tiny amount of time when the bullet is traveling through the barrel and it is past the gas port, but it’s still in the barrel. During this time, there is hot gas traveling through the gas tube into the receiver. As soon as the bullet leaves the muzzle, the gas stops flowing.

If there is too much barrel length after the gas port, then too much gas flows into the receiver and it can causes issues with excessive recoil and wear on the rifle.

If there is not enough barrel length after the gas port, then too little gas flows into the receiver and the rifle may not cycle or may jam.

What is the Correct Gas System Length for my Barrel?

The below chart shows the typical gas system lengths (distance from receiver to gas port) and how the pertain to barrel length. As you can see there are wide ranges here, typically the closer that the barrel length is to the middle of the range, the better it will function (example 14” barrel with a carbine length gas system).


Barrel Length

Port Distance


Less than 10 inches

4 inches


10-18 inches

7 inches


14-20 inches

9 inches


20 plus inches

12 inches


Other factors

It’s worth mentioning that there are factors that can affect the gas system, beyond the length of the system.

Adjustable gas blocks – some gas blocks available that allow you to throttle down the amount of gas that travels through them. For example you can constrict the gas to the point that the rifle is single shot.

Buffer weight – the weight of the buffer can have a big effect on how the AR15 cycles. A heavier buffer will generally require more gas pressure to cycle, so it is possible to moderate the effect of too much gas pressure and smooth out the action by using a heaver buffer, or vice versa with a lighter buffer. Buffer springs can also have an effect.

Ammo type – The weight of the bullet and the powder in the cartridge can make a big difference. A heavier bullet may travel more slowly and cause a longer dwell time, whereas a low-power cartridge may not build enough gas pressure to cycle the action properly.

One Last Tip

If there's anyone that knows the AR-15 platform, it's the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.
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June 21, 2015

Building my first ar15 firing 6.5 Grendel. I have a 20" Shilen match barrel and need to know what length gas tube and hand guard to use. I have a 15" currently.

Bill hilly

November 24, 2015

Who cares, and no it won’t cycle with the carbine buffer and rifle upper. You need a H-14.644 buffer from the mini 30


November 27, 2015

tube length for a DPMS LR308 upper with a 16 inch DPMS barrel?


November 25, 2016

I have a 16" 300AAC rifle. I’m purchasing a suppressor and handload my ammo. I’ve got the loads down to subsonic (around 950 fps). I want to file a form 1 and cut my barrel to 14. How will this affect the gas flow/cycle with using the subs with such little powder. Thank you


February 08, 2017

I have a new build ar15 using m4 setup but a 20" barrel with a rifle length gas system. I have a failure to extract. The bolt doesnt come back far enough to unchamber the spent cartridge. I have ruled out the BCG. The std spring and buffer are used and ive checked the position of the gas block. What could be the problem??