Ask Foghorn: Gas System Length on the AR-15

Tyler Kee asks:

Carbine, mid length, rifle length. What do they mean? Can I run a carbine length on an 18″ barrel? A rifle length on a 16″?

What gives?

Well, you can do whatever you want, but that doesn’t mean the gun will run. Choosing the right gas system is a delicate balance between getting enough gas to cycle reliably and keeping your rifle from beating itself to death. So, which one is right for you? And what is it all about? . . .

Hiram Maxim figured it out first — there’s a TON of wasted energy when a gun goes off, energy in the form of recoil and noise. And if you could capture that energy, you could use it to do something useful like cycling the firearm automatically. Maxim’s design used the recoil of the firearm to reset the gun and load the next round, but it required a massive gun, and wasn’t exactly mobile.

The Vickers gun, with its “muzzle booster,” was one of the first designs to use the force of the expanding gasses rather than the force of the recoil to work the action of the firearm. Similar firearms, such as the M1 Garand and G41(M) would continue to use devices attached to the muzzle to redirect gasses to operate the action, leaving the barrel completely in tact. Browning took it one step further with the B.A.R. or Browning Automatic Rifle, which used a gas system that we would recognize today.

The standard modern gas system (illustrated here thanks to Wikipedia) uses a small hole in the barrel much before the muzzle to redirect some of the gasses to work the action. Bleeding off the gas before the bullet leaves the barrel means the gasses will be MUCH higher pressure than they would be at the muzzle, and allows the action to cycle faster.

There’s a slight issue with using a gas port, especially on the AR-15 platform: barrel length.

In order for there to be enough force to cycle the action, the bullet needs to still be in the barrel when the gas hits the bolt carrier, like a cork in the champagne bottle keeping the pressure up. This is also referred to as “dwell time,” the measure of how long the bullet is in the barrel.

And since the bullet is constantly moving, there needs to be a sufficient length of barrel left so that the bullet leaves the muzzle only after there has been enough gas transferred to the bolt carrier to make the whole thing work. Too much barrel and the bolt is “over gassed,” leaking all over the place making a mess and possibly damaging the system by applying too much force. Too little barrel and there isn’t enough force to unlock the bolt and fully cycle the action.

The original AR-15 came in one barrel length, 20 inches. But as we have shortened the barrel over the years the gas system has needed to be adjusted accordingly to keep the right balance between giving the gun enough gas to cycle the action reliably and keeping the gun from tearing itself apart, since too much rearward force can damage the buffer assembly and other components.

The trick is, as you shorten the barrel, you should move the gas port closer to the muzzle in order to decrease dwell time after the round has passed the gas port. For example, the difference in pressure 9 inches out and 7 inches out from the chamber is almost double, meaning the bullet should probably leave the barrel sooner with the 7 inch port. There are some exceptions to the rule, but for the “proper” gas length that’s the idea.

In order to make things slightly simpler for us consumers, manufacturers have standardized the length of gas systems and provide a guide for which system goes on which length barrel. Here’s ye olde chart:

System Barrel Length Port Distance
Pistol < 10 inches 4 inches
Carbine 10-18 inches 7 inches
Mid 14-20 inches 9 inches
Rifle 20+ inches 12 inches

You can see that the distance between the port and the minimum barrel length increases as you go up the chart – carbine is 3 inches, mid is 5 inches, and rifle is 8 inches. Which keeps things under control.

The problem is with the carbine length gas system. People love to have their stuff “mil spec,” and since the military spec is a 12 to 14 inch barrel a carbine gas system is what they use. Unfortunately, with that whole National Firearms Act thing, civilians generally are restricted to 16 inch barrels. Using a carbine gas system on a 16 inch barrel will work (and work remarkably reliably), but you run the risk of having your rifle wear out quicker. It also leads to some nasty backpressure when using a silencer, which tends to leak out the back of the receiver and straight up your nose. Ask me how I know.

The main exception to the rule is weird calibers. With 5.56x45mm NATO the chart is perfect, but when you have things like .300 AAC Blackout that have different pressures and such it gets complicated. 300BLK is recommended to have a carbine length system for anything 16+ inches and a pistol length system for anything under 16 due to the powder being used, but in general you should listen to the manufacturer’s recommendation for what system to use.

And that, Mr. Kee, is what its all about.

[Email your firearms-related questions to “Ask Foghorn” via guntruth@me.com. Click here to browse previous posts]