What Does an AR-15 Gas Block Do? 

The gas block sits over the gas port on the barrel. When a round is fired, some of the expanding gas is diverted through the gas port. The gas block directs this gas into the gas tube, which then carries it to the bolt carrier group. There are two general categories of gas blocks for the direct impingement system: fixed and adjustable.  

1. Fixed gas blocks are not adjustable in terms of gas flow and feature a single, uniform opening through which gas flows. It is the simplest and most reliable gas block due to its simplicity.  

2. Adjustable gas blocks regulate gas flow by either restricting or venting it. The adjustable gas block typically comes with some sort of adjustable set screw or knob. It allows the user to regulate the gas flow to account for the type of ammunition used, as well as other factors that can affect the weapon's cycling. It is potentially the least reliable gas block due to its complexity and the number of additional parts.

Direct Impingement System

The AR-15 uses a direct impingement gas system, where the gas directly acts on the bolt carrier. The gas block plays a key role in this system by channeling the gas to the carrier, causing it to move rearward and initiate the cycling process (ejecting the spent casing and loading a new round). 

Sizing Gas Blocks for the AR-15

Gas blocks are sized by their interior diameter, typically ranging from .750”, .625”, .875”, to .936”. Measure your barrel's diameter using calipers or refer to the manufacturer’s website for details.

• .750 is the standard size for most barrels, offering the best value since these gas blocks are produced at a larger scale.

• .625 is commonly used for pencil or thin barrels. While these barrels sacrifice rigidity and mass—affecting heat retention—they are the lightest options available. 

What Is a Low-Profile AR15 Gas Block? 

A low-profile AR-15 gas block is smaller and thinner than a standard gas block, allowing it to fit inside free-float handguards with an inside diameter of typically 1.5” or less. These gas blocks are essential when using longer, free-float handguards that extend past the gas port on the barrel. They provide a clean, streamlined look on the rifle. Low-profile gas blocks enable the attachment of various accessories to the handguard without obstruction. 

What Is a Gas Tube? 

An AR-15 gas tube is an essential part of the rifle's direct impingement system. It is a narrow tube that directs high-pressure gas from the gas block on the barrel to the bolt carrier group in the upper receiver. This gas pushes the bolt carrier group backward, starting the cycling process of the firearm. BATTLEARMS® offers stainless steel or black melonite pistol-length gas tubes for superior heat resistance and durability.