Carbine vs. Mid-Length Gas System on a 16″ Barrel

Often times I receive emails and private messages asking why I prefer a mid-length gas system over a carbine gas system on a 16″ barrel or asked which carbine I prefer and why. What is written below is a response to an email, the author was considering a Colt LE6920 and a Bravo Company Manufacturing (BCM) Mid-Length. In my response I explain why I prefer the mid-length gas system over a carbine gas system on a 16″ barrel

Colt 6920 has a 16″ barrel with a carbine length gas system

BCM Mid-Length has a 16″ barrel with a mid-lenght gas system

I own at (3) Colt LE6920’s and (3) BCM Mid-Lengths and have used both at work, in training classes, run and gun rifle matches, etc.

ALL (3) of my Colt LE6920’s have had their barrels changed to Colt 14.5″ SOCOM M4 barrels.

I prefer to use a mid-length gas system on a 16″ barrel.

I do not like a 16″ barrel with a carbine length gas system (same gas system that is on the military M4 carbine)

If I have to use a carbine length gas system I prefer to use a 14.5″ barrel (and I permanently attach a longer flash hider to make the overall length of the barrel 16″).

The 3 most common type of gas systems are:

1) Carbine length gas system – (same length as the US Military M4 carbine) and takes a 7.0″ rail or handguard.

2) Mid-length gas system – the mid-length gas system is 2″ longer than the carbine length gas system (thus the front sight housing and gas hole are 2″ forward of where they are on a carbine length gas system). The mid-length takes a 9.0″ rail or handguard.

3) Rifle lenght gas system – (same gas system on the AR15 or M16A2 / M16A4 with 20″ barrel). The rifle length gas system takes a 12.0″ rail or handguard.

16″ barrel with MID-LENGTH gas system:

16″ barrel with CARBINE gas system:
(pictures property of Bravo Company USA, used with permission)

As you look at both pics, pay close attention to the distance between the front of the receiver and the front sight tower. You can see the mid-length upper has a longer gas system and the front sight housing is 2″ forward as to the location of the carbine length gas system.

When talking about the different gas systems on a 16″ barrel, think about the distance from the gas hole to the end of the barrel. The longer that the bullet is in the barrel after the bullet passes the gas hole, the more gas that is getting pushed back through the gas tube and back into the gas key. The end result is a sharper recoil impulse.

This is why on a 16″ barrel, a mid-length gas system is slightly smoother than a carbine length gas system.

The distance from the gas hole to the end of the A2 flash hider on a 16″ barrel with carbine-length gas system is approximatley 9.5″.

The distance from the gas hole to the end of the A2 flash hider on a 20″ barrel with rifle length gas system is approximatley 7.5″

The distance from the gas hole to the end of the A2 flash hider on a 16″ barrel with mid-length gas system is approximatley 7.5″

The distance from the gas hole to the end of the A2 flash hider on a 14.5″ barrel with carbine-length gas system is approximatley 7.5″

Something to note. The 20″ barrel with rifle length gas system, 16″ barrel with mid-length gas system, and 14.5″ barrel with carbine length gas system all have the same amount of dwell time (distance from the gas hole to the end of the barrel).

The 16″ barrel with the carbine length gas system has a dwell time that is approximtely 1.5″ longer. Thus it’s pumping more gas into the bolt carrier key, forcing it back hard, etc. The 16″ barrel with the carbine length gas system is harder on parts over the long term and you’ll feel slightly more recoil impulse. Not a huge thing, but after shooting all 3 side by side on numerous occasions, I see no need to own a 16″ barrel with a carbine length gas system. I own (18) AR15’s. (2) are short barreled barreled rifles, the rest of my AR15’s are either 14.5″ M4’s with a carbine length gas systems or 16″ Mid-Lengths.

http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system

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