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Gas blocks are an essential part of the AR 15 operating system. The gas block traps gasses that are exiting from the barrel through the gas ports. Once the gas is trapped in the gas block, it is forced down the gas tube towards the upper receiver. From here, the hot gas forces the bolt back, ejecting the spent shell case and loading the next round.
There are two types of gas blocks, fixed and adjustable, with each having their own benefits in different situations.
What Is a Fixed Gas Block?
A fixed gas block is more commonly used on AR 15s and with direct impingement systems. This style of gas block is designed to let in a certain amount of gas from the barrel and cannot be adjusted. The main benefit of this gas system is that the gas block is consistent and reliable.
However, if a rifle is having issues with cycling properly or is over-gassing, then there is nothing you can change with the gas block to fix it. This forces the user to look at other ways to tune the rifle, such as changing buffer assemblies or bolt carrier groups.
What Is an Adjustable Gas Block?
An adjustable gas block operates the same way as a fixed gas block. However, the difference is that the adjustable gas block can be tuned to allow more or less gas into the gas tube. This is typically done by turning an adjustment screw located on the front or side of the gas block. AR 15 adjustable blocks are especially beneficial if you are going to be shooting with a suppressor, have a short barrel, or like to shoot a variety of ammo.
Being able to adjust the gas flow can also help reduce recoil and wear on parts from excessive gas flow. That being said, adjustable gas blocks do cost more than fixed gas blocks. Adjustable gas blocks are also larger and may need a bigger handguard or a handguard specifically designed for use with an adjustable gas block. They also are more likely to experience a failure since there are more moving parts than fixed gas blocks.
Summary
Both fixed and adjustable gas blocks provide the same function on an AR 15 platform weapon. Fixed gas blocks are great for a standard AR 15 that isn’t going to be modified with a suppressor or has a short barrel. Adjustable gas blocks are a good option for anyone with an SBR or AR pistol. They are also excellent options for a rifle that is going to be used with a suppressor. Whether or not you should use an adjustable gas block depends on what the rifle is going to be used for or the overall setup of the rifle.
