FAXON PATENT PENDING ADJUSTABLE .875" 3 SCREW LOW PROFILE GAS BLOCK

How the Faxon Adjustable Gas Block Works

An AR-15 is gas operated: propellant gas leaves the barrel through a small port under the gas block, travels down the gas tube, and drives the bolt carrier rearward. Every barrel is ported for a worst-case load with no suppressor. Add a can, run hotter ammunition, or fit a lighter carrier, and the rifle is overgassed — harder recoil, more gas in your face, faster wear. An adjustable gas block lets you meter that gas to what the rifle actually needs.

Conventional adjustable blocks meter gas with a screw that threads into the gas passage. That screw lives in the hottest, dirtiest spot on the rifle, and carbon packs into its threads until it will not turn. Faxon’s design takes the screw out of the gas path entirely. Gas exits the barrel port into the block’s passage as it would in any fixed block. The difference is that the gas tube is not fixed: it passes through the block and is pinned to a hardened adjustment plate on the rear of the block. Turning the adjustment screw slides the plate, and the tube with it, forward or rearward. That shifts the tube’s inlet hole relative to the block’s gas passage, opening or closing the effective port. Fully forward, the port is wide open. Fourteen clicks rearward, it is closed.

Each click is a real detent: a spring inside the adjustment mechanism presses into notches machined into the screw head, so you feel every setting, the block holds it under recoil, and you can count back to a known setting without a reference mark. The adjustment screw takes a 3/32″ hex key or a flat screwdriver, and because it carries no carbon, the same tool works on it in year five.

What’s Included

Tuning the Gas Setting

Start fully open (the factory setting) and confirm the rifle runs. Then close the block one click at a time, firing a few rounds at each setting, until the bolt fails to lock back on an empty magazine. Open it back up one or two clicks. That is your setting for that ammunition, buffer, and suppressor combination; note the click count so you can return to it. Retune when you add or remove a suppressor or change to a very different load.

Adjust with the rifle unloaded and the block cool. Because the screw never touches gas-port carbon, no cleaning or heat is needed before turning it.

Installation

Tools: small hammer, flat punch, roll-pin punch, and the free 3D-printed assembly fixture (a padded vise works). The gas tube and its roll pin are sold separately.

What Shooters Are Saying

Real feedback from shooters running the Faxon Adjustable Gas Block on their builds.

Faxon .875″ Adjustable Gas Block FAQs

What barrels does the .875″ gas block fit?

Any AR barrel with an .875″ gas journal. That is mostly large-frame AR-10 barrels chambered for bigger cartridges such as 8.6 Blackout, plus heavy and bull profiles that use the larger journal. Each Faxon barrel page shows the gas block diameter it needs. Most standard-profile barrels use a .750″ journal (GBA-LP-750-3), and thinner profiles, such as Faxon’s Pencil and Gunner in 5.56 and .223 Wylde, use .625″ (GBA-LP-625-3). Measure the journal with calipers if you are not sure.

How is this different from other adjustable gas blocks?

Other designs meter gas with a screw threaded into the gas passage, where carbon fouls the threads and eventually seizes the screw. Faxon’s patent-pending design leaves the gas passage alone and moves the gas tube instead: the tube is pinned to an adjustment plate, and the screw slides the plate to open or close the port. The screw never contacts gas-port carbon, so it keeps turning freely.

Are the 14 settings real detents?

Yes. A spring inside the adjustment mechanism engages notches machined into the adjustment screw head. Each click is a positive stop you can feel and count, and the detent holds the setting under recoil without a locking screw or thread locker.

Does it come with a gas tube?

No. The block uses any standard AR gas tube in pistol, carbine, mid, or rifle length, so you can reuse the one you have. The gas tube roll pin comes with the gas tube. Faxon stainless gas tubes are sold separately.

Does it work suppressed?

That is the main reason to own one. A suppressor raises back-pressure and overgasses the rifle; closing the block a few clicks brings cycling speed, recoil, and gas blowback back to normal. Because the adjustment screw stays out of the carbon, it is still easy to open back up when you take the can off.

Will it fit under my handguard?

The block is 1.000″ long, .639″ wide, and 1.493″ tall from the bottom of the block to the top of the gas tube boss on an .875″ journal. The gas tube also sits .080″ higher over the bore than on the .625 and .750 blocks, which keeps it clear of the barrel nut on large-frame short-gas builds. Handguards built for large-frame and heavy-profile barrels clear it; measure the inside diameter of a slim rail before you commit. The adjustment screw is on the rear face of the block, so you adjust it from the receiver side through the handguard’s slots with a 3/32″ hex key.

Can I install it myself?

Yes, with a hammer, a flat punch, and a roll-pin punch. The one extra step compared with a fixed block is that the tube is pinned to the adjustment plate rather than the block body, so set the plate fully forward before you pin it. Faxon offers a free 3D-printable fixture that holds the block square while you drive the pin, and the installation video above walks through every step.

Where is it made and what is the warranty?

Machined from 4140 chromoly steel and flash nitrided in the USA by Faxon Firearms, a family-owned, federally licensed manufacturer in Greater Cincinnati, Ohio. Covered by the Faxon Lifetime Guarantee.

Faxon Lifetime Guarantee. Covers manufacturing and material defects to the original purchaser. Excludes damage from misuse, neglect, improper installation, or modifications. Finish wear from normal use is not a defect. Register your product at faxonfirearms.com/product-registration to expedite service. Full terms: faxonfirearms.com/warranty-privacy-terms.

Patent pending; US Patent Application Publication No. 2025/0277639 A1. Dimensions are nominal from Faxon engineering drawings. Gas tube, roll pin, and barrel not included. Adjustable gas blocks change a rifle’s operating characteristics; always confirm reliable function with your chosen ammunition and configuration before relying on the rifle.

For questions, contact customerservice@faxonfirearms.com.