October 26, 2012
Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
A properly installed gas tube should last for many thousands of rounds, but in the current market where everyone with a barrel nut wrench is building ARs in the basement, we should discuss the relationship between gas tubes and barrel nut alignment.
A slight misalignment between the barrel nut and the carrier key during assembly may cause premature gas tube wear and drag on the bolt carrier. The tip of the gas tube is enlarged slightly and nominal diameter should be .180".
A shoulder is present where the large end transitions to the smaller .170" main tube that can wear away on one side, causing gas leakage and short stroking. Check a suspect tube for this condition visually with a magnifier and verify proper diameter with a micrometer. Tubes that are bent during installation or cleaning may also wear unevenly.
Check barrel nut alignment by stripping the bolt carrier assembly and inserting a #15 drill or .180" gauge pin into the carrier key. Install the carrier and charging handle into the upper receiver and slide it forward until the drill protrudes through a hole in the barrel nut.
If the barrel nut is misaligned, the drill won't be centered and may not even pass through it. Adjust the position of the nut as necessary. I have seen upper receivers that had the gas tube hole drilled slightly off center but fortunately such defects are rare today.
Gas tube replacement is a simple task with the Mark Brown gas tube wrench, available from Brownells, but take care not to bend them on shorter gas systems where they have to be weaseled around a bit during installation. Operators firing full-power 5.56mm NATO spec ammo such as M193, M855 or MK262 may find a worn gas system less problematic than a civilian shooting .223 Rem. commercial fodder at lower pressure levels.
Gauge pin inserted
The .180" gauge pin is inserted into the gas key on the stripped bolt carrier.
The pin protrudes through the barrel nut as the carrier is installed, indicating proper alignment.
The Mark Brown gas tube wrench firmly grasps the gas tube, making removal and installation much easier.