Gas Block Selection

This is not intended to be a comprehensive review of the gas system. Please check out our Gas System Deep Dive for a thorough review.

A Note About Gas System Efficiency

Gas system efficiency is one of the most important considerations for a reliable weapons system. The ability (or inability) of the gas system to hold the gas pumped through it can preclude (or cause) reliability issues and malfunctions.

An efficient gas system makes better use of the gas available to it. In fact, the more efficient the gas system, the more over-gassed a gun may be. However, it is better to have a gun that is over-gassed than one that is over-gassed. You can temper the force from the gas system in several ways. But you can’t get more gas through an inefficient gas system without replacing components.

We cover gas system efficiency in detail in our Gas System Deep Dive.

Gas Block Design

The gas block is a major component of the gas system. It takes gas passing through the gas port and directs it into the gas tube. As mentioned, an efficient gas system is likely to have more gas than it needs. Our philosophy is that it is best to deal with a problem as close to the source as possible. Besides the gas port diameter, the gas block is the best way to modulate the flow of gas into the gas system. Accordingly, some gas block designs allow you to modulate the amount of gas that passes into the gas system.

Fixed Gas Block

All gas blocks are basically a hunk of metal that work to direct the flow of gas into the gas tube and turn it 90 degrees to the rear. That’s pretty much all a fixed gas block does.

Adjustable Gas Block

Adjustable (or tunable) gas blocks add a component of modulation to a fixed gas block. They work on one of two principles, both or which are effective mechanisms for modulating excess gas pressure. By restricting or bleeding off the gas pressure at the gas block, you can send just enough gas to through the gas system to cycle the gun. And if things change (change in ambient temperature, switching to lower/higher pressure ammo, worn parts, a dirty gun, under-lubrication, etc.), you can always dial the gas flow up or down.

Restriction Gas Block

In a restriction gas block, the adjustment mechanism can be incrementally or continuously adjusted to block off the flow of gas through the gas block. The restriction method effectively reduces the size of the orifice through which the gas flows to get into the gas tube. This restriction limits the volume of gas that can enter the system (as if you had a smaller gas port). This is how the most adjustable gas blocks work.

In its simplest form, a restriction (or constriction) gas block mechanisms consists of a double set screw: first screw is used to block some of the gas flow and the second is used to lock the first screw in place. More sophisticated designs often feature a detent-captured adjustment screw that can be adjusted in increments.

Benefits

Simpler design. Fewer parts to fail.

Drawbacks

Restriction screw is susceptible to erosion and seizing.
Depending on the design, the detent mechanism is often susceptible to fouling and lockup of the adjustment screw or seizing of the detent.
Double set screws can be a challenge to lock into the right setting, because the second set screw often rotates the first out of the adjusted position.

Bleed-Off Gas Block

In a bleed off gas block, the adjustment mechanism is an incrementally adjustable system allows the venting of gas. This mechanism does not restrict the flow of gas into the gas system; instead, it diverts a portion of the gas through the vent, effectively reducing the system pressure.

We are only aware of one manufacturer of a bleed-off gas block: Superlative Arms. Their DI Adjustable Gas Block is an amazing feat of engineering. The device can be set to either restrictive or bleed-off modes. We’ve covered the restrictive mechanism but will add that SupArms’ external detent design prevents fouling and lockup of the detent mechanism. The bleed-off mode of the device vents excess gas toward the muzzle of the gun through a channel that runs parallel to the adjustment screw.