Match the gas system to the barrel length. Pistol gas suits barrels under about 10 inches, carbine gas 10 to 16 inches, mid-length 14.5 to 18 inches, and rifle gas 20 inches and up. On a 16-inch 5.56 barrel, mid-length is the popular softer-shooting choice and carbine works too. 300 Blackout barrels usually use pistol gas, even at 16 inches.
AR-15 gas system length chart
| Gas system | Gas port (from the receiver) | Typical barrel lengths | Gas tube (maker listings) |
|---|---|---|---|
| Pistol | About 4 in | Under 10 in, and most 300 Blackout | 6.625 to 6.75 in |
| Carbine | About 7 in | 10 to 16 in | 9.75 to 9.8 in |
| Mid-length | About 9 in | 14.5 to 18 in | 11.75 in |
| Intermediate | About 10 in | 16 to 20 in | Not standard (Wilson Combat: 13.25 in) |
| Rifle | About 12 in | 20 in and up | 15.125 to 15.25 in |
How the AR-15 gas system works
The AR-15 is direct impingement. When the bullet passes the gas port, a small hole in the top of the barrel, some of the gas behind it flows up through the gas block, back down the gas tube and into the gas key on the bolt carrier. That pushes the carrier back, unlocking the bolt, extracting and ejecting the case. The buffer spring then drives everything forward to chamber the next round.
Why the length has to match the barrel
Two things change with the port's position. The farther the port is from the chamber, the lower the pressure when gas reaches it, because the bullet has already traveled farther down the bore. And the barrel left in front of the port, called dwell, sets how long the gas keeps pushing before the bullet exits. The original 20-inch rifle has about 8 inches of dwell; the closer a build is to that balance, the softer it tends to run.
| Barrel | Gas system | Port | Dwell (barrel minus port) |
|---|---|---|---|
| 10.5 in | Carbine | About 7 in | About 3.5 in |
| 14.5 in | Carbine (M4) | About 7 in | About 7.5 in |
| 16 in | Carbine | About 7 in | About 9 in |
| 16 in | Mid-length | About 9 in | About 7 in |
| 18 in | Intermediate | About 10 in | About 8 in |
| 20 in | Rifle | About 12 in | About 8 in |
A 16-inch carbine-gas barrel has a port close to the chamber, where pressure is high, which is why it feels harsher than the same barrel with mid-length gas. Very short barrels have little dwell, so they need the port close in: a 10.5-inch barrel usually runs carbine or pistol gas.
Signs the gas system is wrong for your rifle
- Over-gassed (port too close for the barrel, or a suppressor added): harsh recoil, brass thrown far forward, faster wear on the bolt and extractor, and sometimes the bolt outrunning the magazine.
- Under-gassed (port too far for a short barrel, or weak ammo): short-stroking, failures to eject or lock back on the last round.
A common rule of thumb is that brass landing around the 3 to 4 o'clock position means the rifle is gassed about right. You can tune a slightly over-gassed rifle with a heavier buffer or an adjustable gas block.
Common questions by barrel
- 16-inch 5.56: mid-length for a softer rifle; carbine is more forgiving of weak ammo.
- 14.5-inch: carbine (the M4 setup) or mid-length.
- 18-inch: mid-length or intermediate, depending on the barrel maker.
- 300 Blackout: usually pistol gas, even on 16-inch barrels, because the round runs at lower pressure. See 300 Blackout parts.
Matching parts
The gas tube must match the gas system (see AR-15 gas tubes), the gas block must fit the barrel journal (gas block sizes), and the handguard must clear the block (handguard length for your gas system). Still choosing a barrel? See the barrel length guide. Compare gas blocks and tubes and barrels by price; the AR-15 builder flags a gas system that doesn't match your barrel.
Frequently asked questions
What are the AR-15 gas system lengths?
Pistol, carbine, mid-length, intermediate and rifle, with the gas port about 4, 7, 9, 10 and 12 inches from the receiver.
What gas system is best for a 16-inch barrel?
Mid-length is the popular choice for a softer 16-inch 5.56 rifle; carbine gas works too and is more forgiving of weak ammo.
What is dwell time?
How long gas keeps pushing on the carrier after the bullet passes the port, set by the length of barrel in front of the port.
What happens if the gas system doesn't match the barrel?
Too short a system for the barrel over-gasses the rifle: harsh recoil and faster wear. Too long a system on a short barrel can short-stroke and fail to eject.
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 tube made for the barrel's gas system; a mid-length tube is about 11.75 inches.
