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AR 15 - Gas Blocks


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AR-15 Gas Blocks Frequent Asked Questions

When you pull the trigger on an AR15, and the bullet travels down the barrel, the rifle will use 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, the casing is ejected, and the buffer and buffer spring compress, slowing the bolt to a stop and returning it to the forward position.

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

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

There are many styles of gas blocks, which can impact the performance and aesthetic of your rifle. To get the most out of your AR, choose a high-quality gas block that suits your rifle’s design and purpose.

The most popular gas blocks are a standard fixed design, commonly found on most commercial AR15s.

These gas blocks are very pragmatic and trustworthy, but they are a jack-of-all-trades and master of none. No matter what ammo, suppressor, buffer or different weight BCG you use, the same amount of gas passes into the gas tube—often more than necessary.

The justification behind this is that over-gassing allows the system to continue working, even when it gets dirty. The downside is that over-gassing increases recoil and wear, though many would say this is negligible outside extreme circumstances.

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. Adjustable gas blocks are also NOT a fix for a barrel with an oversized gas port. You can limit the flow, but you’re putting a bandage on a problem without actually fixing it.

In the end, adjustable gas blocks are best for versatility, while standard gas blocks are best for ease of use.

If you’re considering a free-float handguard, a low-profile gas block is mandatory.

Low-profile gas blocks are shorter and thinner than standard blocks, allowing fitment with thin free-float handguards. For that reason, many handguard kits will include a low-profile gas block.

Low-profile gas blocks can also shave an ounce off the front of your rifle, making it easier to hold your sights on target for longer.

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

There are two common gas block sizes. A .750 diameter 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 your barrel’s advertised spec.

Different ARs will have different gas system lengths, and your gas tube will need to match. Longest to shortest, gas tube lengths include Rifle, Intermediate, Mid-length, Carbine, and Pistol.

When choosing a barrel, you’ll also choose the gas system length. It is important to understand the relationship between gas system length and barrel length, as your selection will impact reliability, recoil, and overall performance.

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, offering a longer dwell time for smoother operation. Rifle length gas systems work best when used with long barrels, between 18” and 20”.

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

By reducing pressure, you put less stress on the internal components, 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.

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 compromise between a carbine length and rifle length gas systems.

Mid-length gas systems are a popular choice for 14.5”-16” barrels, offering reduced recoil impulse when compared to a carbine-length system.

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 stabilize before the bullet passes the gas port. At higher pressures, the bolt carrier group will cycle faster, improving function when the rifle is dirty.

Carbine-length gas systems are ideal for short barrels between 10.5” to 13.7”, but you can also find 14.5” to 16” barrels with carbine gas systems as well.

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

For ultra-short ARs, the pistol-length gas system boasts a mere 4” port length. Many rifles chambered in 300 BLK with 16” barrels will also use the short pistol length gas system to maximize reliability with subsonic ammunition.

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

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.