Short answer: the gas block sends gas from the barrel into the gas tube. Pick the one that fits your barrel diameter and handguard, then secure it properly.
Clamp-on
Clamps around the barrel and is easy to fit. Check for proper alignment before tightening.
Set-screw
Held by screws into a dimple or detent on the barrel. Use thread-locking compound as directed.
Pinned
Pinned in place for a solid fit on barrels that are drilled for it. Fitting needs the right tools, so many builders have a shop do it.
Match the diameter
Barrels come with different gas-journal sizes. Confirm yours, and see the gas system compatibility guide for length pairings.
What a gas block does
The gas block sits on the barrel over the gas port. It catches the gas that bleeds off as the bullet passes and sends it into the gas tube, which drives the bolt carrier group back to cycle the rifle. If the block is loose or not lined up with the port, the rifle can short-stroke or fail to cycle at all.
Match the barrel first
Before you pick a type, measure or confirm the barrel’s gas journal diameter. The common sizes are .750 inch (low-profile, the most popular for free-float handguards), .625 inch (some lightweight barrels) and .936 inch (the standard front sight base profile). A block must match the journal or it will not seat correctly.
Clamp-on
A clamp-on block wraps the barrel and is tightened with screws on the clamp itself. It needs no drilling or dimpling and can usually be installed at home with basic tools. Make sure it is aligned with the gas port and use thread locker on the screws.
Set-screw
A set-screw block locks onto the barrel with screws that bite into dimples or the barrel surface. It is simple and common, but it only holds as well as the screws do, so use thread locker and check it after the first range trip.
Pinned
A pinned block is secured with taper pins through the block and a mating hole in the barrel. It is the most permanent option and is common on factory and high-end builds. Installation usually calls for drilling or a gunsmith, so it is best if you want a build you rarely take apart.
Which one to choose
- For a first build or a home installation, a clamp-on is the easiest.
- For a simple, low-cost build with dimpled barrel, a set-screw block works.
- For a long-term or duty-style build, choose pinned.
Whatever type you choose, line it up with the port, check the gas tube length matches your gas system, and function-test the rifle before relying on it.
Keep building
Browse AR-15 parts, read the custom AR-15 build guide, or let our Palmetto, FL shop help with gunsmithing and custom AR builds.
Shop AR-15 parts at BMTC USA, a gun shop and gunsmith in Palmetto, FL.