All About AR-15 Gas Port Sizes

The importance of the AR-15's gas port is often overlooked. When there's talk about gas systems and tuning a rifle, the conversation usually focuses on gas blocks, buffers, and gas tubes. There's little mention of gas port size.

Maybe you're building an AR-15, and need to verify you've got the right port diameter for your chosen gas system and barrel length. Maybe you've just bought a new upper or barrel, and you want to make sure your gas system's specifications are up to, well, spec.

So, what's the optimal gas port size? We've got all the answers below.

Why Gas Port Size Matters

The AR's gas port size -- the diameter of the port hole in the barrel -- is, in fact, one of the most important specifications. Figuring out if your barrel has a port that's too big or small can be the key to diagnosing and fixing an AR that's got issues.

Rather than spend money on expensive things like an adjustable gas block, a new barrel, or a different buffer, you might simply need to adjust your port size to get your gun running well.

How Gas Port Size Affects Performance

In addition to dwell time, it's the diameter of the port that determines how much gas makes its way into the bolt carrier group to cycle the gun. If the gas port's too big, the BCG will receive too much gas.

This makes the AR run rough. You'll deal with excess felt recoil, and your upper receiver will suffer from rapid fouling, unnecessary heat, and early wear.

But if the port's too small, the BCG will get too little gas. This causes the bolt to short stroke. This often results in failures to eject spent casings, and failures to feed new rounds from the magazine. Both of these things cause jams.

AR-15 Gas Port Sizes

Thanks to the AR-15 community's obsession with data, we've got hundreds of gas port measurements for virtually every barrel length and gas system. You can view the raw data by clicking that link, but we've taken the time to sort through it all, do the math, and find the optimal gas port diameters for various setups.

Gas Port Sizes for 300 BLK Pistol-Length Gas Systems

All Barrel Lengths

• Gas port diameters range from 0.082" to 0.110".
• The average gas port diameter is 0.095".

Note: Because data is limited, we cannot differentiate between gas port diameters for different 300 BLK barrel lengths. All 300 BLK barrels are equipped with pistol gas systems.

Gas Port Sizes for 5.56/.223 Pistol Gas Systems

• Gas port diameters range from 0.062" to 0.125".
• The average gas port diameter is 0.089".

Note: Because data is limited, we cannot differentiate between gas port diameters for different 5.56/.223 barrel lengths equipped with pistol gas systems.

Gas Port Sizes for Carbine Gas Systems (By Barrel Length)

10.5" Barrels

• Gas port diameters range from 0.064" to 0.086".
• The average gas port diameter is 0.074".
• A diameter of 0.055" is recommended for dedicated suppressor use.

11.5" Barrels

• Gas port diameters range from 0.063" to 0.084".
• The average gas port diameter is 0.070".
• A diameter of 0.051" to 0.067" is recommended for dedicated suppressor use.

12.5" Barrels

• Gas port diameters range from 0.059" to 0.079".
• The average gas port diameter is 0.069".

13.7" Barrels

• Gas port diameters range from 0.055" to 0.059".
• Available data is too small to obtain an average; any port between these ranges is acceptable.

14.5" Barrels

• Gas port diameters range from 0.062" to 0.071".
• The average gas port diameter is 0.065".

16" Barrels

• Gas port diameters range from 0.053" to 0.088".
• The average gas port diameter is 0.067".
• *The ideal gas port diameter is 0.062".

Note: The ideal gas port size for a 16" barrel with a carbine-length gas system is likely smaller than the calculated average size (0.067"). To obtain the ideal size (0.062"), we eliminated port diameters greater than 0.071" from the pool of measurements, and recalculated the average.

Why? Because 0.071" is the maximum port diameter found on carbine-gassed 14.5" barrels, which have shorter dwell times.

Therefore, port sizes above 0.071" on carbine-gassed 16" barrels will likely cause over-gassing, due to this barrel and gas system combination's longer dwell time.

Gas Port Sizes for Mid-Length Gas Systems (By Barrel Length)

12.5" Barrels

• Gas port diameters range from 0.060" to 0.094".
• The average gas port diameter is 0.081".

13.7" Barrels

• Gas port diameters range from 0.072" to 0.087".
• The average gas port diameter is 0.078".

13.9" Barrels

• Gas port diameters range from 0.073" to 0.083".
• The average gas port diameter is 0.077".

14.5" Barrels

• Gas port diameters range from 0.072" to 0.082".
• The average gas port diameter is 0.077".

16" Barrels

• Gas port diameters range from 0.063" to 0.087".
• The average gas port diameter is 0.076".

Gas Port Sizes for Rifle-Length Gas Systems (By Barrel Length)

16" Barrels

• Gas port diameters range from 0.076" to 0.115".
• The average gas port diameter is 0.089".

18" Barrels

• Gas port diameters range from 0.075" to 0.108".
• The average gas port diameter is 0.088".

20" Barrels

• Gas port diameters range from 0.081" to 0.123".
• The average gas port diameter is 0.092".

Note: As the rifle-length gas system's barrel length increases, so too does the average gas port diameter in the data above. This is counterintuitive because, as barrel length increases, dwell time increases.

A longer dwell time would logically necessitate a smaller gas port diameter. Yet in the available data, we found Colt's government-contracted 20" M16 barrels have gas port diameters of 0.093".

So, it's likely the ideal port diameter for shorter barrels (16" and 18") equipped with rifle-length gas systems is likely greater than the calculated averages. This discrepancy is likely due to only small data sets being available for 16" and 18" barrels.

"What If My Gas Port Size is Off?"

Even if your barrel's gas port diameter is different from the reported average, you shouldn't worry -- unless, of course, you're dealing with issues like excess felt recoil, jams, or a short-stroking BCG. Then you should check to see whether your gas port's size is contributing to these cycling issues.

Need help inspecting your gas port? See our instructions: How to Build an AR Upper. Just follow the steps in reverse to remove your handguard and gas block from your barrel. Then measure your exposed gas port to verify its size.

If your gas port is too small, the fix is easy: Just drill the port so it's slightly larger and more closely matches the reported average. If your gas port is too big, it's time to invest in an adjustable gas block to reduce the amount of gas flowing through the port to the BCG. Or, you can use this as an excuse to upgrade to a new barrel.