Along with the buffer tube system, one of the lesser-known components of an AR build kit is the gas system, specifically a component known as the gas block.
This is unfortunate, as the gas block is critical to the success of the gas system, which is responsible for capturing some of the propellant gasses generated during, siphoning them off to the bolt carrier group’s gas key, and cycling the action.
Without a functioning gas system, a rifle with a gas impingement operation, such as an AR, would not be capable of autoloading action. Complicating matters is the fact that there’s more than one type of gas block.
Here’s what to know.
What Is a Gas Block?
Easily forgotten since the gas system is typically attached to the barrel as a part of the build kit and therefore seen as an integral component, the gas block is nonetheless a special and unique part.
When you fire the AR, propellant gasses expand in the chamber and bore, accelerating the bullet forward in the barrel.
When the bullet passes the gas port, which is basically a small hole in the barrel, some of the pressure can escape through this port and then through the part known as the gas block, and from there to the gas tube, which sends the gasses back to the gas key on the bolt carrier group.
A common configuration, especially in military-style rifles, is known as an FSB gas block. This stands for “front sight base” gas block and serves not only as the gas block itself but also as the front sight. This is one of the strongest and most reliable gas block types because they are commonly permanently pinned to the barrel.
However, there are many different types of gas block that can be used in the construction of an AR-style rifle. Some of these will be covered here.
The Different Types of Gas Blocks (and Should You Upgrade?)
As stated, one of the most common types of gas blocks is the FSB gas block, which is permanently pinned to the barrel and which is therefore not modular. It is however physically very durable, reliable, and consistent. It’s the preferred style of gas block for shooters that prefer a classic look and feel with their AR rifles but it doesn’t offer a lot of flexibility.
Another type of gas block is known as a low-profile gas block. Basically, a low-profile gas block operates just like a standard gas block except it is machined to tighter tolerances to have a lower profile against the barrel (hence the name). Because of this configuration, narrower and slimmer handguards can be mounted around the barrel since the gas block has a smaller footprint and won’t get in the way.
All in all, low-profile gas blocks are probably the most common and popular type of gas block on the market today. They can accommodate most types of handguards and have become more or less a standard. There is a good chance any given AR built kit will come with a low-profile gas block, so if you aren’t sure, check the included parts list before buying.
Another type of gas block that is growing in popularity is known as an adjustable gas block. While there is more than one type of adjustable gas block configuration, the basic premise is that the operator can tune how much gas is allowed through the block and then captured and siphoned off to the bolt carrier group.
An adjustable gas block gives operators significant control over the gas system, allowing them to scale back over-gassed rifles and gas-up undergassed rifles. This enables the operator to make changes to cycling speed and reliability without changing the buffer spring or weights.
Moreover, being able to tune the gas system of an AR-style rifle gives the operator fine control over action time, recoil control, and more, making it easier to tune performance when shooting different rounds, such as subsonic rounds or those loaded with hotter powder charges and heavier bullets.
Furthermore, adjustable gas blocks are fairly easy to tune via a single set screw that controls how much gas is captured; once installed, they offer the operator easy control over rifle performance and reliability, adding to their growing popularity.
Shop AR Build Kits and Gas System Components Here
If you’re getting ready for a home-build project with an AR build kit and want to swap out the stock gas block for an adjustable one so you can fine tune the system, take a look through our collection of AR parts so you can customize your build. We sell gas system components, including low-profile gas blocks, adjustable gas blocks, and models that are low-profile and adjustable. For more information, check out the previous link or get in touch with us at Sales@MCSGearup.com.