Carbine Vs Mid Length Gas System. Which One You Should Choose?

If you are building an AR-15 or changing out some parts on one you already have, I’m sure you’ve run into this dilemma before. Is a carbine or mid length gas system better for my AR?

In this article, we will go over some of the key facts of the gas system, and make some very simple recommendations about which gas system is better for you.

Carbine or Mid Length Gas System

Carbine or Mid Length Gas System

Gas System Basics

The gas system of an AR-15 is what allows the weapon to cycle. Once you fire the weapon, gas is generated by the chemical reactions of the propellant of the cartridge. This gas pushes the projectile forward, but also work to cycle the bolt. 

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It is actually pretty simple how this is accomplished. The gas system has a small hole to vent gasses that is near the front sight post of the weapon. The vent hole is connected to a gas tube, that goes back into the receiver.

Once the bullet passes the vent hole, the gas will enter this hole and flow through the gas tube until the bullet exits the barrel. Once the gas moves back into the receiver, it provides the power for the bolt carrier group to chamber the next round.

However, after the bullet exits the barrel, the gas won’t all vent through the small vent hole anymore. Most of it will vent through the end of the barrel.

As you can imagine, this all happens pretty fast.

What's the different between Carbine vs Mid Length Gas System

Carbine length gas systems are shorter than mid length gas systems. What this means is that the vent hole is closer to the receiver on a carbine length gas system. In turn, that means that the vent hole is further from the end of the barrel.

So…. What Does That Mean?

The length of the gas system affects the cycling of the weapon. If the bullet is past the vent hole but in the barrel for a longer amount of time, more gas will enter the vent hole. So, if the vent hole is closer to the barrel, less gas will enter the gas tube.

In a carbine length gas system, there is a greater distance between the end of the barrel and the vent hole. This means that more gas will enter the gas tube when compared to a mid length gas system.


On a standard 16 inch or an 18 inch barrel, we recommend a mid length gas system.

The reason for this is that a carbine length gas system will allow more gas to enter the tube, which will increase recoil, and will cause additional wear on the internals of your weapon.

On a 14.5 inch or shorter barrel, we recommend a carbine length gas system. With this shorter barrel, there is less space between the vent hole and the end of the barrel, so the correct amount of gas will enter the gas tube.

If you are using a 20 inch barrel, a rifle length gas system should be used.

Harvey Specter

"Sometimes hunting is the only thing that makes sense"

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