AR-15 Gas Port Diameters: What's Best?

We recently covered why dwell time is so important for your AR's ability to cycle reliably. The length of your gas system determines dwell time, which determines how much gas flows into your bolt carrier group to cycle the action. Too much gas causes excess recoil and wear and tear. Too little gas makes for an unreliable AR.

Gas port size can influence whether the right amount of gas is getting to the BCG, too. We're covering AR-15 gas port diameters to help you make sure your barrel's set up to cycle your action with the right gas flow.

What is Gas Port Size?

Those referring to the AR's gas port size are simply referring to the port's diameter. In other words, how big or small the hole in the barrel is where gas flows through and into the gas block, and gas tube.

What is The Most Common Gas Port Size?

The most common AR-15 gas port diameter, no matter the barrel length nor calibe, is 0.076". This median -- not average, just the most common individual diameter -- gas port diameter was calculated by collecting and comparing the gas port measurements of more than 400 barrels. You can find the full list of data here.

Is this 0.076" gas port size ideal?

Yes, but only for 5.56/.223 barrels with carbine and mid-length gas systems. Pistol- and rifle-length systems have different optimal diameters based on the data, but we'll cover that next.

This 0.076" diameter is what most barrel manufacturers skew towards, including major defense contractors like Knight's Armament, and top-tier barrel makers like Noveske, LaRue Tactical, Daniel Defense, Colt, Bravo Company USA, and Criterion.

Does gas port diameter change with gas system length?

Yes. In finding the median gas port diameters for each gas system length, we found significant differences. For each gas length, we calculated the median diameters of all barrels optioned with only that gas length, and found these different median gas port sizes:

• Pistol Length: 0.095"
• Carbine Length: 0.070"
• Mid Length: 0.078"
• Rifle Length: 0.092"

It's interesting to note that on the extreme ends, pistol- and rifle-length gas systems have the largest gas port diameters. Carbine and mid-length systems have diameters closer to the universally typical diameter of 0.076", with mid-length systems measuring the closest, at 0.078".

What are the smallest and largest gas port sizes?

The smallest gas port size we found across all barrel and gas system lengths was 0.051". This was found on a Sionics 11.5" barrel optioned with a carbine-length system. Other barrels with similarly small port sizes -- those measuring between 0.052" and 0.060" -- were exclusively configured with carbine gas systems, and all barrels measured between 11.5" and 12.5".

The largest gas port sizes were found on primarily pistol- and rifle-length gas systems across an extreme spread of barrel lengths, with examples ranging from as short as 5" to as long as 20". The largest gas port diameter, totaling 0.143", was found on an ultra-short "micro-length" gas system on a 5" barrel.

Can gas port diameter change with caliber?

Yes, but only to a degree. Barrels chambered in .300 AAC Blackout tend to have larger gas port diameters -- at least 0.080" to as large as 0.110" -- to ensure reliable cycling when firing subsonic ammunition.

Other calibers, like .308 Winchester and various intermediate rifle calibers like 6.5 Grendel and 6.5 Creedmoor, tend to have gas port sizes closer to the universal median of 0.076".

Help! My gas port size is weird!

So long as your AR-15 cycles reliable, you needn't worry about whether your gas port diameter's close to the typical size, or if it seems larger or smaller. Barrel manufacturers don't just arbitrarily pick a diameter -- they conduct extensive testing to ensure their barrels' gas systems, including the length and port, work well with the intended cartridge.

With that said, if your AR-15 seems over- or under-gassed, it may worth inspecting your gas port and measuring it with a digital micrometer to determine its size. If your AR's under-gassed, increasing the diameter of the port is an easy way to remedy the issue. If your rifle's over-gassed, confirming that you have an oversized port is a great way to avoid wasting money on unnecessary fixes.

Read our guide on dwell time and gas system lengths to learn more about whether your AR might be over- or under-gassed!