I’ve said a few times that various tasks in building an AR-15 range in complexity and mechanical aptitude. They range from basic plumbing to small engine repair.
Fitting on a new barrel is pretty much similar to the first: it’s a pipe (barrel) with a threaded collar (barrel nut).
However! Like that under-the-sink job, there are still tools, supplies and tricks that help to make sure it’s been done right.
So, without further adieu, here are my essential parts—before, during and after—of any AR-15 barrel install.
Part 1 (Before): Shop Essentials
I put what might be the most often overlooked and important item first on this list because it’s, well, overlooked and important: A gas tube alignment-check tool.
One can be bought or built. It’s a rod with a 0.180-inch diameter that fits into the carrier key and replicates the gas tube. Make one out of a 4-inch straight section cut from the body of an old gas tube (don’t include either end segment).
Beyond a sturdy bench and vise, other essential shop tools include a fixture to secure the upper receiver. There are different takes on fixtures and I prefer the “block”-style.
On these, the receiver lugs fit into the block and quarter-inch diameter pins—the same size as the pivot and takedown pins on the lower— and slips through to secure the upper.
Fixtures that go inside the upper (in place of the bolt carrier assembly) and work off of the barrel extension bolt lugs are increasingly popular (like the excellent Geissele Automatics Reaction Rod).
The reason I prefer the block-style is because they retain access to the inside of the upper to make gas tube alignment checks (using that cool tool).
You’ll also need a wrench handle (more in a bit) and head to fit the barrel nut. All I’ve encountered require a half-inch drive. The wrench head can vary greatly depending on the barrel nut that goes with your handguard rail.
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. (You can also buy it from Cheaper Than Dirt!) 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!
Part 2 (During): Install Properly
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.
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 to answer why the breaker bar? Because loosening with a torque wrench will damage its mechanism.
Now onto using that torque wrench: Set it at least a couple of foot-pounds under desired torque, and that’s because wrenches are calibrated from the center of the handle to the center of the drive socket.
Most any wrench head will extend above the drive center, and that increases leverage. There’s a formula to be perfectly precise, and it’s shown below.
The universally-recommended torque setting is 35 ft.lbs. That figure came from military armorers manuals, such as Colt’s M16A1 Armorers Depot Maintenance and Repair Manual, which I use for reference. Now. 35 ft.lbs. is not magic, it’s just minimum.
I have some question even if it’s minimum, but that’s another story, and brings up also another question: do you really need a torque wrench? That honestly is a doggone good question.
The cautiously-correct side of me says “yes,” but the commonsensical side of me says, “not really.”
Lemme explain: I’ve yet to see a USGI-pattern barrel nut not get to at least 35 ft.lbs. when the nearest scallop is coming into, but not yet reached, alignment with the gas tube receptacle in the upper receiver. Thread timing just doesn’t allow it.
Usually, it’s more than 35 to get aligned. It is, in my view, more important to have a torque wrench to know when you’ve hit the right level if you’re not using a USGI-pattern nut.
Part 3 (After): Align the Gas Tube
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.
The reason this is important is that any binding in the gas tube will displace the carrier. I’ve seen some on the net claim it really doesn’t matter, but, folks, ask any and all precision-oriented builders and they will tell you that it absolutely matters for an AR-15 barrel install.
If you encounter a stubborn case, like when there’s about a half-scallop to go to get lined up, stop and repeat the tighten/loosen process another 3-4 times. There’s only so much it can do, but it can very often do enough…
Note: The preceding is a specially-adapted excerpt from Glen’s book, The Practical AR15.
Have you done an AR-15 barrel install before? Share any tips you have in the comments below!