How to Install Ar-rifle Gas Blocks

The year: 1977. I was a young Cav trooper serving in Jimmy Carter’s Army. We lined up in front of the armorer’s cage with our weapons cards and traded them for a rifle, 5.56MM M16A1. Grab your gear and mount your vehicle. Our weapons were produced by well-known defense contractors: H&R, Colt, and GM. We had faith in our small arms, and they were generally reliable. Some, like the 1911 pistol, M3 SMG and M2 .50 cal. Machinegun, had been proven in multiple conflicts.

The gas tubes on those M16A1s were pinned into a front sight housing, which served as a gas block/front sight/bayonet mount. The design was simple and robust, as military weapons should be. This front sight was considered part of the barrel assembly and it was not removed for any reason.

Fast forward 40 years, and the rifles we have today, although based on the original Stoner design, offer a dizzying array of options to the casual observer. Barrels are available in rifle, carbine and pistol lengths in various weights and with different-length gas systems. Stoner-pattern rifles are gas operated and the gas that drives them is tapped from the barrel through a gas port. A “gas block” serves as a conduit between the barrel port and the gas tube, directing gas back through the upper receiver to the gas key on the bolt carrier, driving the carrier rearward and unlocking the bolt. The gas block on your modern sporting rifle performs the same function as those M16A1 front sights did 40 years ago. It may look different, but is functionally identical.

Gas blocks are available in many styles. This article will help you choose the part you need for your build and demonstrate various installation techniques.

The first part I select for a build is the barrel, and then I assemble a rifle around it. Admittedly, the gas block is one of the last components I consider. Choosing one is as simple as running through a checklist. Factors to consider are listed and examined below.

Barrel diameter at the gas port: First generation black rifles like the M16A1 and Colt SP1 sporter were equipped with lightweight “pencil” barrels measuring 0.625" in diameter at the gas port. In the 1980s, the M16A2/AR15A2 came along with a barrel that was heavier at the muzzle end, measuring 0.750". For all but the heaviest target barrels, 0.750" is still the standard today. If you are running a really heavy varmint or target barrel you may need a 0.875" or 0.936" gas block. Check your barrel diameter before ordering parts.

Adjustable or fixed gas port: Is the rifle we are building intended for duty or defensive use? I would lean toward a fixed gas block on a defensive rifle. Simple is good and less prone to cause problems when you don’t need them. On the other hand, adjustable gas blocks can smooth out rifles that are over gassed or frequently fired with a sound suppressor. Recently, I worked on a rifle from a major manufacturer sporting a 20-inch barrel and carbine-length gas system. It benefited noticeably from an adjustable gas block.

Barrel gas ports are a compromise size to allow the weapon to function with a wide variety of ammunition, from 5.56 NATO (hot) to imported steel-cased ammo loaded to moderate pressure levels. If you feed your carbine a steady diet of NATO pressure ammo, the gas can probably be turned down a bit while maintaining reliable function, but remember, we’re mainly talking about 5.56mm weapons that aren’t known for much recoil, so don’t get anal about adjustable gas blocks unless you just like to tinker with stuff. A standard gas block will work fine on most rifles.

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