Why are the AR-15 gas system lengths? More importantly, why does it matter? This is one of the most common questions I get asked by shooters building their first AR-15.
If you don’t know the appropriate length of the gas system for your build, don’t worry. I got you covered. That is what I’m here for, right?
Today I will be absolutely giving you a talk about size and why it matters.
In AR-15s, anyway.
And we’ll also be talking about which barrel length you should use if you are building your first or tenth AR-15—no time to waste. Let’s get straight to the point.
What Does The Gas System Do?
I’m glad you asked.
I’ll make it super simple. The gas tube on your AR-15 (Or any other Direct Impingement firearm) makes the bolt carrier group go back and forward. If you want a functioning rifle, you need the right gas system.
Or things can start to go bad fairly quickly.
Think of the gas system of a rifle as you would the gas system in your car. And the bolt carrier group to be the motor.
When a round is fired, the gases that aid in propelling the round are pushed through the barrel and out the muzzle.
But, on the way out, some of the gases are brought back through a piece of the barrel called the gas block. The gas block is a bridge between the barrel and the gas tube. And the gas tube floats over the barrel.
The gas tube used the gas from the round to push the bolt carrier back into the buffer and cycle a new round into the chamber.
What If I Have Gas System Issues?
Then your rifle will have issues with cycling. You’ll suffer stoppages and failures that could be more expensive than Natalie McLennan.
Or, you’ll just break your rifle apart from too much. But why does it break apart?
There is this thing called the dwell time. It’s how long the round is still in the barrel after it passes the gas port.
If there is too much barrel for the round to go through after the gas port, then you will suffer from over-gassing, which can damage your rifle.
You may suffer from cycling issues if there isn’t enough barrel left after the gas port.
But if you get a barrel from a company that knows what they’re doing, the port will be in the correct spot. And you won’t have to worry too much.
How To Use Ejection As A Sign
Think of a normal wall clock.
If you are getting ejection around the 3:30 and 4 o’clock positions, your rifle is gassed properly.
If the casing ejects at the one to 3:00 o’clock position, it is over-gassed. The 4:30 and 5 o’clock means your rifle is under-gassed.
Keep it in the 3:30 and 4 o’clock range. Straight out to the side is always best.
What Can Cause Gas Issues?
Ammo and suppressors are the most common culprits for a rifle with gas issues. You usually see these issues rear their head when you’re shooting underpowered ammo.
A perfect example is sub-sonic rounds.
Sub-sonic rounds are loads designed to eliminate that super-sonic crack that you hear when a round goes off near you. Sub-sonics fly under the speed of sound and thus have less pressure behind them.
And if you’ve ever shot 5.56×45 subsonic rounds with a suppressor, you know what I’m talking about. The round won’t cycle, and you’ll have an ejection failure.
But you could use it like a bolt action. Just kidding.
And to keep it short, suppressors eat up a lot of the gas from the shot, which is why the under-gassed rounds have issues with ejection and things of that sort.
Which is why .300 Blackout was created. I’ll be speaking about that more in my next article.
Okay, Let’s Talk Length
There are four primary gas lengths that you will find in the AR-15 world. There may be more, and you can even have custom lengths, but we’ll stick with the most common lengths for now.
Here’s a small graph of the lengths, barrel lengths, and the barrel sizes they are meant for.
|Barrel Length||Gas System Length||Port Distance||Best Buffer Weight|
|10 Inches or Less||Pistol||4 inches||H3|
|10-18 Inches||Carbine||7 Inches||Carbine or H2|
|14-20 Inches||Mid-Length||9 Inches||H2|
|18-20 Inches or More||Rifle||12 Inches||H2 or H1|
If this doesn’t seem too self-explanatory, let me talk about this a bit more in-depth. I’ll include a rifle for each sub-category just in case you learn visually as I do. Some of this might not make sense since we just talked about dwell time. I’ll get to that in a second.
These are usually seen on your AR-Pistols. They’re short. And the gas escapes the barrel just four inches away from the chamber.
These set-ups are best with an H3 buffer weight to limit that wild recoil and violent cycling.
These are also seen on some AR-Pistols but are usually used on SBR (Short Barreled Rifle) through 18-inch barrel categories. The gas port is seven inches away from the chamber. These typically use a Carbine buffer.
These can be used on SBR through Rifle length barrel. The port is nine inches away from the chamber and uses an H2 buffer. My BCM Recce MCMR is a mid-length and uses an H2 from VLTOR. Slight flex.
For the musket fans out there who love their Stag Arms 20” rifles. These gas lengths are used on 18-20+ inch barrels, and the gas port sits 12 inches away from the chamber. These are usually run with H1 or H2 buffers.
Running A Different Size Gas System?
You may be asking why you see shorter gas tubes on longer or shorter rifles.
For example, you can run a carbine-length gas tube on an 18-inch barrel. But why would you do that if you could just run a mid-length of rifle length?
Running a longer-length gas system on a shorter barrel delays the speed of the bolt carrier cycling. Obviously. Since there is more length for the gases to go through. What this also does is lowers the pressure when the round is extracted. It prevents early wear and tear, which is always a plus.
You can see competition shooters running adjustable gas blocks alongside longer gas systems, all in pursuit of the balance between reliability and light recoil for faster shots.
What is the best barrel length for a mid-length gas system?
14-20 Inch barrels can use mid-length gas systems the best. But it all depends on what you are going for with your AR build.
What are the different AR-15 gas system lengths?
1. Pistol Length
2. Carbine Length
4. Rifle Length
Can an AR pistol have a carbine length gas system?
Yes, an AR Pistol can have a carbine length gas system.
AR gas system lengths may seem a bit confusing if you don’t know too much about it, but after reading this, you should better understand what the different lengths are used for and why one might be used instead of another.
Remember, different things like the load you’re firing and whether or not you are running a suppressor will have an effect on how your rifle functions. Just keep an eye out, and remember that 3:30-4 o’clock is always best.
See you on the range, fine shooter.