Short answer: match the gas system to the barrel length. Pistol gas for barrels under about 10 inches, carbine for 10 to 16 inches, mid-length for 14.5 to 18 inches, and rifle length for 20 inches and up. On a 16-inch 5.56 barrel, mid-length is the popular soft-shooting choice; carbine works too. 300 Blackout barrels usually use pistol gas, even at 16 inches.
Gas system length chart
| Gas system | Gas port (from receiver) | Typical barrel lengths | Gas tube |
|---|---|---|---|
| Pistol | About 4 in | Under 10 in (and most 300 Blackout) | Pistol length |
| Carbine | About 7 in | 10 to 16 in | Carbine length |
| Mid-length | About 9 in | 14.5 to 18 in | Mid-length |
| Intermediate | About 10 in | 16 to 20 in | Intermediate |
| Rifle | About 12 in | 20 in and up | Rifle length |
Why the length matters
The farther the gas port is from the chamber, the lower the gas pressure when the bolt starts to move. A gas system that's long for its barrel runs softer and is easier on parts; one that's too long for the barrel may not cycle reliably. That's why mid-length is a favorite on 16-inch barrels, and why short barrels need pistol or carbine gas.
The gas tube must match the gas system, and the handguard must clear the gas block: see handguard length for your gas system.
Parts that fit
Shop mid-length barrels, carbine-gas barrels, gas blocks and tubes, and read gas systems explained for more depth.
Frequently asked questions
What gas system is best for a 16-inch barrel?
Mid-length is the popular choice for a soft, reliable 16-inch 5.56 rifle; carbine gas works too and is a little more forgiving of weak ammo.
Carbine or mid-length gas?
Mid-length runs softer on barrels 14.5 inches and longer. Carbine suits 10 to 16-inch barrels and cycles a wider range of ammo.
What gas system does 300 Blackout use?
Usually pistol length, even on 16-inch barrels, because 300 Blackout runs at lower pressure.
Does the gas tube have to match?
Yes. Use the gas tube length that matches the barrel's gas system.
