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AR 15 - Gas Blocks


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AR-15 Gas Blocks Frequent Asked Questions

With an AR-15, when the trigger is pulled and the bullet travels down the barrel, the rifle uses some of the hot gas to cycle the action.

This begins with those hot gasses entering the gas block from the gas port of the barrel, traveling down the gas tube, pushing the bolt carrier back, and extracting the empty casing.

As the bolt carrier group moves rearward and the casing is ejected, the buffer and buffer spring inside the stock assembly compresses, slowing the bolt to a stop and returning it to the forward position.

As the bolt returns forward, the magazine lines up a new round to be fed into the chamber and the bolt catches it while returning to the forward position.

The gas system is composed of the gas block and gas tube. There isn’t much you can do with a gas tube, it’s a steel tube and the way it affects the rifle is generally tied to its length.

The gas block is the component that fits over the small hole on the top front of an AR-15's barrel. It’s the valve that controls how much hot gas gets directed back into the receiver.

The magic happens when you install a higher-quality gas block — even a non-adjustable one.

The gas block on most factory AR’s are fixed. By only allowing a certain amount of gas through, it's optimized for one out of an infinite number of setups. No matter what ammo, suppressor, buffer or different weight BCG you use, the same amount of gas is allowed into the gas tube.

— typically more than necessary.

The justification behind this is that overgassing allows the system to continue working even when it gets dirty. The downside is, this can lead to increased wear and recoil

However, a high-quality non-adjustable gas block is best for duty AR builds, it allows your gun to run safely and reliably without having to worry about gas settings.


An adjustable gas block allows you to tune the amount of gas entering the tube. This means the bolt can receive the perfect amount of force, you can feel less recoil, and your gun will be easier to clean.

You can change ammo loads, use a suppressor, and change buffer weights too.

A drawback to adjustable gas blocks is they require cleaning more often to operate at their best.

Because there are many AR’s like it, but this one is yours, efficiency is everything. Adjustable gas blocks let a direct impingement system work smoothly, while standard gas blocks are best for ease of use.


If you’re like me and love free float handguards, a low profile gas block is a must for you.

Their lower height means they allow for the handguard to go over the gas block. This isn’t possible with a standard height gas block.

They are also a massive 1 oz. lighter too. Yippee!

Gas blocks come in different sizes to fit different barrel diameters.

There are two common gas block sizes. A .750 is standard for barrels of a medium profile, while .625 is a common size for pencil barrels. Simply shop for the gas block that fits the barrel of your gun.

The other half of the gas system is the length. Longest to shortest, they are: Rifle, Mid-length, Carbine, and Pistol.

For this article, we are referencing the more common direct impingement system with a gas tube.

Gas system length and effects don’t change between Direct Impingement or piston-driven.

The longer the system, the further away from the gas port and gas block will be located down the barrel.

Where the gas port is located has a drastic effect on a rifle’s reliability and felt recoil, and long-term parts durability.

Gas Port Length~12"
Gas Tube Length~15"
Ideal Barrel Length18" - 20"

The rifle length gas system places the gas port roughly a foot down the barrel. Allowing the bullet to travel quite far before passing the gas port; offering a longer dwell time and providing better accuracy.

By extending the dwell time, pressure within the chamber and barrel are reduced as well. Lower gas pressure means the bolt carrier group will move more slowly translating to lower felt recoil.

This lower pressure puts less stress on the internal components of the firearm than other gas systems, increasing the life of many small parts.

With less energy to cycle the rifle, it may be more sensitive to dirt and fouling, requiring more regular maintenance.

Rifle length gas systems also work best when used with longer barrels.

Gas Port Length~9"
Gas Tube Length~11.75"
Ideal Barrel Length~14.5"-16"

The mid-length gas system places the gas port roughly 9” down the barrel, providing a nice compromise between a carbine length and rifle length gas systems.

Mid-length gas systems are extremely popular for their generally smooth recoil and reliability.

Gas Port Length~7"
Gas Tube Length~9.75"
Ideal Barrel Length~10.5"-16"

With the gas port placed 7” down the barrel, there is very little time for gas pressure to be reduced before the bullet passes the gas port.

With more high-pressure gas available, the bolt carrier group can cycle faster, and therefore harder; improving the rifle’s ability to function when it is extremely dirty.

This leads to a more violent action of the carbine gas system with sharper recoil than longer options, as well as a slight increase in the wear and tear.

Gas Port Length~4"
Gas Tube Length~6.75"
Ideal Barrel LengthLess Than 10.5"

For those times when your barrel is just too short, there is the pistol length gas system. This is typically found on AR pistols and registered short barreled rifles with 10.5-inch barrels or shorter.

Many rifles chambered in 300 BLK with 16” barrels will also use the short pistol length gas system to maximize reliability with subsonic ammunition. The short dwell time and high pressure allow the firearm to function reliably.

Bleeding the hot gasses so close to the chamber will lead to increased fouling, higher operating temperatures, and increased wear over carbine length systems.

Now, of course there is more to the whole story than just the gas system.

Felt recoil can be drastically altered with the use of muzzle brakes or using lighter or heavier buffers.

Simply put, every component you choose for your rifle is a compromise, and your gas system is no different. Choosing the perfect gas system for your build is simply finding the compromise that best suits what you want in your rifle. Besides, no one is saying you can only have one AR.

If you have any questions, don't hesitate to send us a message at info@primaryarms.com or call us at 713-344-9600.