AR-15s

3 Essential Parts of Any AR-15 Barrel Install

ar-15 barrel install

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.

AR-15 install barrel
For my AR-15 barrel install projects, I like a receiver mounting fixture that allows easy access to the interior of the upper.

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).

Geissele Reaction Rod
Another commonly popular and good fixture is the Geissele Reaction Rod. This uses the lugs on the barrel extension to secure the upper.

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.

AR-15 Barrel Install
Essential tools in an AR-15 barrel install.

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!

ar-15 anti-seize
Anti-seize is another very important and often overlooked part of any AR-15 barrel install. It’s a wise precaution, and an asset to easier installation.

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.

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.

ar-15 barrel install wrench
Tighten, loosen. Work the barrel nut back and forth a few times. This helps! And, if you have aluminum barrel nut, scrap it for a steel one if it’s possible.

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.

wrench head diagram
A wrench head will throw off the setting on a torque wrench. Use this formula to offset it. It’s usually around 5 ft.lbs.

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

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!

ar-15 barrel install alignment
Don’t settle for less than perfection in gas tube alignment. Your gun will not shoot as accurately as it should if you don’t get this right.

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.

ar-15 barrel install tools
Very important tool in any AR-15 barrel install: a gas tube alignment fixture. Buy one or make one, but use one!

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.

A lot.

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!

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (23)

    1. White lithium is not a hi-pressure lube and it won’t stand the heat either just use Aeroshell MS33 . It’s easy to find and not that expensive.

  1. I’ll admit I’m a fan of Glen’s writings and have all his books (even his latest), so I might be biased. Regarding a couple of comments above: the point about even barrel extension contact with the receiver is valid, but two big considerations are that it’s not really needed for most builds that aren’t going to be for precision rifle competition, and to do so requires actual shop tools and techniques (I think it was one of Joe Carlos’s AR build articles in American Gunsmith that discussed the problems created or exacerbated with solutions that are chucked into a drill). I’m not trying to say those are the “final answers as rebuttals” to the concerns raised, but as dialogue as to why I think the question is valid, but not generally applicable.

    As to Sam’s question, barrels do in fact wear out. And yeah, it takes a lot of shooting. Some high volume competitors only have builders build their guns, and some like to do most/all the work themselves. As to your philosophy, I’m like that and actually so is Glen. I recommend you check out some of his writing to get a sense of his approach–it’s very measured, he’s transparent and brings a wealth of experience to the table. His build guide to competitive rifles might be a bit overwhelming, but his latest about America’s Rifle is easily digestible and wide-ranging. Just a thought.

  2. Good info. One tip though, if you keep the barrel nut wrench at a 90 degree angle to the torque wrench then there is no need to calculate a new torque since you are neither increasing or decreasing the length of the torque arm. Waaaaaaay easier than doing the math!

  3. I was surprised to not to see a lapping procedure of the upper receiver to the barrel contact point.
    I have never seen a perfect witness line completely around around the receiver surface when first checked, the amount lapping required can result in a shim needed for gas tube alignment – unless hand guard design doesn’t require this.

  4. I have a kinda noobie question. Unless you are building from scratch, why would you want to change barrels? It’s just a guess on my part but I don’t think barrels wear out. And if you can afford to run enough ammunition thru it to wear it out, you probably can afford to buy the best high dollar mod job available. Btw, I subscribe to the if it ain’t broke don’t fix it, school magazine.

  5. Do not use anti-seize if it contains graphite and most of them do it will react with the aluminum and corrode the upper receiver. Use AeroShell 33ms or it’s replacement AeroShell 64.

  6. 10/5/19: Re the great article about AR15 Barrel install:
    The instructions for using the Torque Wrench in a Straight-Out Position
    can be easified bu turning the Torque Wrench 90 Degrees from Straight-Out and then using the actual needed Torque Setting!
    No Calculations Required.

    Stewart Warner used to have a Slide Tool that You could easily set and read the correct Torques either Straight-Out or turned 90Degrees.

    Hope this helps, gene

  7. If you put the crows foot at 90 degrees to the torque wrench, and not straight out, B=0, and viola, NO MATH required. The torque wrench setting will be the actual applied torque.

  8. The diagram that illustrates the torque conversion formula shows the torque head in line with the torque wrench. Why not attach the head perpendicular to the torque wrench? Then, the distance between the center of the handle to the center of the head will be virtually unchanged. Not that it matters though. The specs are, a very broad, 35 to 80 foot pounds. I agree that you should torque to 35 and then, tighten just enough past that to align the gas tube.

  9. These are some very useful points, but I can’t help but point out that barrel extension fitment inside the upper receiver and lapping the face of the upper receiver are also important considerations in such discussions on accuracy. Barrels whip as the projectile moves through the barrel. Small gaps in the axial fitment of the extension and receiver faces not square to the extension affect the consistency of the barrel whip and affect shot placement. Perhaps not a huge amount, but for the guy hoping to wring out that last .25MOA, they do make a difference. Ideally, the barrel extension should be “snug” or even to the point of needing to be tapped into the upper receiver. I have personally come across fitment with as much as .01″ slop between extension and receiver ID. Needless to say, this is terrible, but technically still in spec. In such cases, you should consider swimming the extension evenly, or alternately, putting a light coat of high temp loctited around the extension evenly and clamping it to cure with the barrel nut.

  10. I use a 5/32 drill bit shank as an alignment tool sometimes (some of the gas tube holes vary in size) . Letter size bits can sometimes tell you when you are coming into alignment, starting with ones somewhat smaller than the hole.

  11. Great article!!! I work in the aviation industry, where most things are torqued. I ofter read about trq’ing the barrel nut to 35 ftlbs. However, most commonly used tools are adding length to the tool, thus creating an improper trq. on nut. Nice add with the equaision, by the way!

  12. Thanks for the great information. I will be sure to pay closer attenion to gas tube alignment.
    I have a question about using a copper type anti seize. If using a steel or aluminium barrel nut with an aluminium upper reciever, should there any concern with the possibility of galvanic corrosion?
    Thanks.

  13. Okay, so you don’t use headspace gauges and an unassembeled bolt to check for proper headspacing ?
    That should be the first thing you do !!!

  14. It should be noted that the use of copper based anti seize compounds (as presented in the story) could be risky as copper and aluminum do not play well together. Copper would not be a problem for the barrel nut but the upper receiver being made of aluminum could allow the copper to form corrosion on the threads. This could result in damage to the upper receiver threads as well as great difficulty removing the barrel nut for future modifications. I always feel safer using a nickel based anti seize compound for this threaded connection of the barrel nut (available at NAPA).

  15. Very well written general purpose instructions.
    Often, precision rifle builders want a snug or even a light press fit of the barrel extension to the upper. I have used shim and I have used Rocksett to tighten any fitment issues. My current 3gun competition rifle uses Rocksett and it is very accurate, sub-MOA.

  16. good info.. BUT. Anti seize IS NOT recommended. read the tech specs. anti seize between 2 different causes many problems

  17. Thanks for sharing these tips. I have been thinking about doing a 80% build and my local Gunsmith, who builds ARs and AKs from scratch quite regularly, has offered to give me input if I run into problem. Still all these little tips I run across are helpful and provide insight. Thanks.

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