Gas Tubes

What a Gas Tube Does, and Why the Material Matters

A direct-impingement AR has no piston. Gas leaves the barrel through a port under the gas block, travels down the gas tube, and enters the bolt carrier through the gas key, where it drives the carrier rearward. The tube is a fixed part of the rifle’s timing: its length is set by the gas system the barrel was drilled for, and its inside stays hot and carbon-coated for the life of the gun.

That is why every Faxon gas tube is made from 304 seamless stainless. 304 keeps its corrosion resistance through repeated heat cycles and needs no coating, so there is nothing to wear through where the tube slides in and out of the gas key on every shot. Seamless tubing is drawn from solid rather than rolled and welded, so the wall is one continuous piece with a smooth bore and no seam to corrode or collect carbon. It is bright and uncoated, which also makes it easy to inspect for bends and buildup.

The Four Lengths

Barrel lengths are typical, not a rule. A 16″ barrel can be carbine or mid-length gas depending on how it was drilled; a 12″ 8.6 Blackout is pistol gas on an AR-10. Go by the gas system listed on the barrel.

Installing It

With the gas block off the barrel, drive out the old roll pin with a 1/16″ punch and pull the tube. Orient the new tube with the gas hole facing the barrel and the flared, slotted end toward the receiver, slide it into the block until the pin holes align, and drive the new pin flush. Set the block on the journal with the tube passing through the barrel nut into the receiver; it should enter the gas key freely. If it binds, rotate the block until it does, then torque the set screws. Full steps and a pairing guide are on each product page.

Gas Tube FAQs

How do I know which gas tube length I need?

Match the barrel, not the rifle. The gas system is set by where the port is drilled, and every Faxon barrel page lists it: pistol, carbine, mid, or rifle. If the barrel is already installed, measure from the center of the gas block to the receiver face: about 4″ is pistol, 7″ carbine, 9″ mid, 12″ rifle.

Are AR-15 and AR-10 gas tubes the same?

Yes. Both platforms use the same gas tube diameter, the same gas key, and the same four lengths. A Faxon gas tube fits either.

What does seamless mean, and why does Faxon use it?

Seamless tubing is drawn from a solid billet, so the wall is one continuous piece. Welded tubing is rolled from strip and welded down its length, leaving a seam that is a different microstructure and often a bead inside the bore. In a gas tube that runs hot and full of carbon every shot, a seamless wall has no weak line to corrode and no rough spot for carbon to build on.

Why 304 stainless?

304 is an austenitic stainless that keeps its corrosion resistance through repeated heating, which is exactly what a gas tube sees. It needs no coating, so there is nothing to wear off at the gas key, and it is the alloy family the mil-spec gas tube is made from.

Does a Faxon gas tube come with a roll pin?

Yes, every length ships with a new installation roll pin. Use a new pin each time; the old one deforms on removal.

Will these work with the Faxon adjustable gas block?

Yes. The adjustable block meters gas by sliding the gas tube itself, and it uses a standard tube in any length. Buy the tube that matches your barrel’s gas system.

When should I replace a gas tube?

When a clean, lubricated rifle with good ammunition short-strokes or will not lock back; when the tube is bent, kinked, or scored at the receiver end; when carbon will not clear; or after very high round counts when the gas-block end shows erosion. It is a wear item and an inexpensive fix.