Why an Adjustable Gas Block, and How Faxon’s Works
A direct-impingement AR 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 — weak ammunition, no suppressor — so a rifle running hot loads, a suppressor, or a lightweight carrier is overgassed. That shows up as harder recoil, gas in your face, a carrier that outruns the magazine, and faster wear on the bolt, extractor, and buffer. An adjustable gas block lets you meter gas to what the rifle actually needs, and change it when you add or remove a can.
Conventional adjustable blocks meter gas with a screw threaded into the gas passage. That screw lives in the hottest, dirtiest spot on the rifle, and carbon packs its threads until it will not turn. Faxon’s patent-pending design takes the screw out of the gas path entirely. The gas tube passes through the block as it would in any fixed block, but it is pinned to a hardened adjustment plate on the rear of the block instead of to the block body. Turning the adjustment screw slides the plate, and the tube with it, forward or rearward, which shifts the tube’s inlet hole relative to the block’s gas passage and opens or closes the effective port. The screw only ever touches the plate.
Each of the 14 settings is a real detent: a spring inside the adjustment mechanism presses into notches machined into the screw head, so you feel every click, the setting holds under recoil and heat cycling, and you can count back to a known setting without a reference mark. Adjust with a 3/32″ hex key or a flat screwdriver, from the receiver side, through the handguard.
Choose the Right Journal Size
The gas journal is the turned-down section of the barrel the gas block clamps to; measure it with calipers, not the barrel itself. Every Faxon barrel page lists the gas block diameter it needs. All three blocks share the same 1.000″ length, .639″ width, adjustment mechanism, and three 10-24 set screws; only the bore, body diameter, height, and (on the .875) gas-tube height change.
| Journal |
Part Number |
Typical Barrels |
Body Ø |
Height |
Tube Height |
MSRP |
| .625″ | GBA-LP-625-3 | Thinner profiles, such as Faxon Pencil and Gunner in 5.56 / .223 Wylde | .774″ | 1.288″ | .625″ | $89 |
| .750″ | GBA-LP-750-3 | Most standard and mid-weight AR-15 / AR-10 profiles — the most common size | .899″ | 1.350″ | .625″ | $89 |
| .875″ | GBA-LP-875-3 | Large-frame AR-10 barrels such as 8.6 Blackout, and heavy profiles | 1.024″ | 1.493″ | .705″ | $92 |
Tube height is bore center to gas-tube center. The .875″ sets the tube .080″ higher so it clears the barrel nut on large-frame builds with short gas systems. Height is bottom of block to top of the gas-tube boss — the clearance your handguard needs.
Specifications (All Sizes)
| Adjustment | 14 tactile, spring-detented settings, fully open to fully closed |
| Adjustment Tool | 3/32″ hex key or flat screwdriver |
| Mounting | Three 10-24 set screws, bottom of block |
| Bore Tolerance | +.0015″ / −.0000″ |
| Length × Width | 1.000″ × .639″ |
| Material | 4140 chromoly steel (block, adjustment plate, and screw) |
| Finish | Flash nitride |
| Gas Tube | Any standard AR gas tube, any length — sold separately |
| Patent | Patent pending, US 2025/0277639 A1 |
| Made In | USA — Greater Cincinnati, Ohio |
| Warranty | Faxon Lifetime Guarantee |
What’s in the Box
Gas Block
Adjustment plate, detent spring, and screw assembled.
Three 10-24 Set Screws
Installed in the block.
Not Included: Gas Tube
Any standard tube; roll pin ships with the tube. Faxon gas tubes ›
Tuning in Four Steps
1. Start fully open (factory setting) and confirm the rifle runs.
2. Close one click at a time, firing a few rounds at each, until the bolt fails to lock back on an empty magazine.
3. Open back up one or two clicks. Note the click count.
4. Retune when you add or remove a suppressor or change to a very different load. No cleaning or heat needed first — the screw carries no carbon.
Installation: Overview and Step-by-Step Video
Tools: small hammer, flat punch, roll-pin punch, and the free 3D-printable assembly fixture (a padded vise works). The one difference from a fixed block is that the gas tube pins to the adjustment plate, so set the plate fully forward before you pin it. Then: slide the tube in from the rear, align the roll-pin holes, drive the pin flush with the plate, fit the block over the journal and the tube into the upper, bore-scope the block to the barrel’s gas port, and tighten the three set screws.
Installation tutorial (1:09)
What Shooters Are Saying
Real feedback from shooters running the Faxon Adjustable Gas Block on their builds.
Complete the build
Gas Tubes and Faxon Barrels
The blocks ship without a gas tube because they take any standard AR tube you already own. Faxon stainless gas tubes come in pistol, carbine, mid, and rifle lengths with the roll pin included. Pairing the block with a Faxon barrel? The barrel page tells you which journal size to order.
Gas Tubes › Free Assembly Fixture STL › All Gas Blocks & Tubes ›
Adjustable Gas Block FAQs
What makes Faxon’s adjustable gas block different?
The adjustment screw never enters the gas path. It moves an adjustment plate that the gas tube is pinned to, sliding the tube to open or close the port, so carbon cannot foul the screw threads and seize the adjustment. Conventional designs thread the metering screw directly into the gas passage.
Which journal size do I need?
Measure the gas journal on your barrel, or check the barrel’s product page. .750″ is the most common AR-15 and AR-10 size. .625″ is for thinner profiles, such as Faxon Pencil and Gunner barrels in 5.56 and .223 Wylde. .875″ is for large-frame barrels such as 8.6 Blackout and heavy profiles.
How many settings are there, and are they real clicks?
Fourteen, from fully open to fully closed. A spring inside the mechanism engages notches in the adjustment screw head, so each setting is a positive detent you can feel and count, and it holds under recoil without thread locker or a lock screw.
Does it come with a gas tube or roll pin?
No. The block uses any standard AR gas tube in any length, so you can reuse yours. The roll pin ships with the gas tube. Faxon stainless gas tubes are sold separately.
Is it for suppressed or unsuppressed rifles?
Both. Suppressed, close the block a few clicks to tame the extra back-pressure, blowback, and carrier speed. Unsuppressed, tune for reliable lock-back with your ammunition and the softest recoil that still runs. Because the screw stays clean, switching between the two is a few clicks with a hex key, not a cleaning job.
What tools do I need to adjust it?
A 3/32″ hex key or a flat screwdriver. The screw is on the rear face of the block, so you reach it from the receiver side through the handguard slots.
Will it fit under my handguard?
The blocks are 1.000″ long and .639″ wide, and stand 1.288″ (.625), 1.350″ (.750), or 1.493″ (.875) tall from the bottom of the block to the top of the gas-tube boss. That clears the large majority of free-float handguards; check inside diameter on very slim rails, especially with the .875.
Where are they made?
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.
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. Faxon Lifetime Guarantee covers manufacturing and material defects to the original purchaser; full terms at faxonfirearms.com/warranty-privacy-terms.
