What Size Should the Gas Port Be on My AR-15 Barrel?
Gas port size depends on both barrel length and gas system length. For a typical 16-inch mid-length barrel, the gas port should be 0.070 to 0.073 inches. Shorter barrels with carbine gas typically need 0.078 to 0.093 inches, while rifle-length systems on 18- to 20-inch barrels use 0.086 to 0.093 inches.
Why It Matters
The gas port is a small hole drilled in the barrel that allows propellant gas to enter the gas system and cycle the action. If it is too small, the rifle will short stroke and fail to cycle. If it is too large, the rifle will be overgassed, causing harsh recoil, premature wear, and violent extraction. Gas port size is set by the manufacturer and is not easily changed after the fact, making it important to choose the right barrel from the start.
The Detail
Gas port diameter specifications by gas system and barrel length:
Pistol-length gas (barrels 7.5 to 10 inches):
- Port diameter: 0.070 to 0.086 inches
- Shorter barrels within this range need larger ports due to less dwell time
Carbine-length gas:
- 10.5-inch barrel: 0.078 to 0.083 inches
- 14.5-inch barrel: 0.070 to 0.078 inches
- General range: 0.078 to 0.093 inches
Mid-length gas:
- 14.5-inch barrel: 0.073 to 0.076 inches
- 16-inch barrel: 0.070 to 0.073 inches
- General range: 0.070 to 0.078 inches
Rifle-length gas:
- 18-inch barrel: 0.086 to 0.089 inches
- 20-inch barrel: 0.086 to 0.093 inches
- General range: 0.086 to 0.093 inches
These specifications vary by manufacturer. Premium barrel makers like Criterion and Triarc tend to use smaller, more precisely sized gas ports and recommend adjustable gas blocks for fine-tuning.
Build Impact
You generally cannot change the gas port size on an existing barrel without specialized tools. If your barrel is overgassed, the practical solution is an adjustable gas block rather than modifying the port. When purchasing a barrel, check the manufacturer's specifications. Premium barrels with smaller gas ports paired with adjustable gas blocks give you the most tunable setup. Budget barrels tend to have oversized gas ports for reliability with a wide range of ammunition.