Cheaper than Dirt.

If it’s not USGI-pattern (that has 20 “scallops” around its circumference), most provide a head to match their nut. But I’ve seen a few I just had to wing it with, which often meant resorting to a large adjustable wrench to fit the wrench flats on the nut.

To fit the drive, ask for a “crows-foot” attachment at an auto parts store.

If your barrel nut is USGI-pattern, get a wrench head that fits over and into as much of the nut circumference as you can. My favorites give 360-degree contact, but most are half that (or less). And make sure the doggone thing is securely fitting into those scallops!

I hold in against the wrench head when I work the wrench handle to keep the head from slipping.

Longer wrench handles are better than shorter ones. Longer makes it easier to make those often-necessary small-but-high-effort nudges easier to feel, and to initiate. I use both a torque wrench and a breaker bar, and the latter is because of the next important item.

Anti-seize! This auto-parts store item is a critical component.  It’s a copper-based lubricant intended exactly for what we’re doing here—it prevents galling.

Galling is abrasive wear from the friction that occurs when metals that are compressed against one another are put into motion.

If the compressive forces are high enough between the surfaces (and they sure can be), the friction can create heat sufficient to weld the materials together and that then removes material from one surface and places it onto the other. Not good!

To get the operation going, clean off all associated surfaces (inside of the upper and outside of the barrel extension). Slip the barrel extension into the upper (there’s a pin on the extension and a notch in the upper that line up).

Put an even coat of anti-seize around the circumference of the upper threads (I use a flat artist’s brush) and thread on the barrel nut. Using something other than a torque wrench, tighten the nut down firmly—give it a good pull—and back it off.

Repeat that three or four more times: tighten it to snug-plus and back it off.

Why?

Because that helps mate the surfaces by facing down any small imperfections (which will usually be on the upper). The anti-seize allows this tactic.

The tighten/loosen procedure is compressing tiny bits of metal, and the lube is preventing galling, as well as make it easier to loosen.

Now for more about that “alignment with the gas tube receptacle in the upper receiver. ” That is absolutely critical, or it is if you want your AR-15 to shoot as well as it can.

With a USGI-pattern nut, that means one of the scallops has to be dead center in the gas tube receptacle in the upper so the gas tube isn’t touching the nut—not even a little bit. With another style barrel nut, it might not mean a thing.

The point is that if there is an opening on the nut that should align with the gas tube receptacle, it has to align!

That is now when and how the gas tube alignment tool really helps. Remove the bolt from the bolt carrier group, insert the tool in the carrier key, and slip it into the upper. There should be a gap 360-degrees around the tube. It’s a tiny gap, but it’s a gap.

Ultimately, final check it with the gas tube itself, and the test then is that the gas tube should rattle—move freely all directions.

This isn’t an evolution I’ve performed.  I’d like to think I could watch before doing it.