AR-15 Barrel Installation

BY Herschel Smith
1 month, 2 weeks ago

I have never replaced a barrel myself, but one day I might, and if you have, he gives some useful tips.

Here’s what you’ll need: upper receiver fixture; barrel nut wrench head; 1/2-inch drive torque wrench; 1/2-inch drive breaker bar; anti-seize; gas-tube alignment tool. And, of course, a sturdy vise.

To secure the upper receiver I prefer a “pins”-style block, because it gives easy and clear access to the upper interior to make gas tube alignment checks easier. On these, the receiver lugs nestle into the block and 1/4-inch pins, just like the takedown and pivot pins on the gun itself, fix the receiver in place.

The wrench head can be variable. If you’re working with a GI-standard-style barrel nut (has the spiny scallops all around it), there are many to choose from. I prefer one that engages full-circle; most are half-circle. The more scallops engaged, the more secure the connection is under pressure. Otherwise, the head depends on the particular barrel nut that shipped with the handguard rail you chose. If it’s proprietary, most will provide a wrench head along with the package.

Torque wrench: bigger the better, I say. More leverage is easier to operate. Speaking of leverage, adding the extra length of a wrench head requires an offset adjustment. Torque wrenches are accurate and calibrated at a point halfway up the handle (there’s a mark there) to the center of the drive socket. Measure from the center of the handle to the center of the drive socket. Measure then from the center of the drive socket to the center of the wrench attachment. Add those together.

Now divide that total length figure by the length of the handle to drive center. Multiply that by the torque level you want to apply. The answer is then where to set the torque index. For instance, if you have a 15-inch long wrench, center of handle to center of drive, and attach a wrench head that extends 2 more inches beyond the drive socket center, and you’re looking for that standard 35 foot-pounds — 15 + 2 = 17. 15 divided by 17 = 0.882. 0.882 x 35 foot-pounds = 30.87. So, dial the wrench handle to “31.”

He said that backwards (divide the length of the handle to drive center by the total to get the ratio, not the other way around), although his example calculation is fine.  Make sure to read carefully and think it through.

About the gas-tube alignment tool: this can be either ready-made or homemade. I use one that’s a solid steel rod 0.180-inch diameter. If you make one from an old gas tube, for a more precise fit take a section from the gas tube body, not the portion that fits into the carrier key, and make it 3-4 inches long. This is an important tool to have.

The anti-seize comes from an auto parts store and is a copper-based lubricant that does what it suggests it does: it prevents galling. It also protects against dissimilar metals corrosion, and that includes aluminum against steel. It makes the barrel nut easier to install and then uninstall.

Get the upper fixed into whichever fixture you choose. Get your barrel. Wipe down the soon-to-be-mated surfaces to remove any residue or debris. There’s a pin on the barrel extension that fits into a corresponding slot on the threads surrounding the barrel extension receptacle in the upper. Seat the barrel into this receptacle. It usually will slip back and seat fully. If it doesn’t, don’t worry, because the barrel nut will seat it.

Next, get the anti-seize. Put a thin, even coat on the receiver threads. A flat artist’s brush works dandy.

Now get a barrel nut. I use one that I keep separate just for this next job. It’s a USGI-pattern nut that fits snugly and securely into my wrench attachment. The reason I have one that’s separate is because I sometimes “save” the supplied handguard nut until final install (I encounter proprietary aluminum wrench heads that get loose with use, as well as others that aren’t super-solid in engagement with the nut).

Fix the wrench head onto a breaker bar. Why? A torque wrench should not be used to loosen. Run the barrel nut on and tighten it firmly. Give it a yank. Then back it off, and snug it down again, firmly again. After experience you get a calibrated forearm, and you’ll get a feel for how much equals what. I take it to at least the 35 foot-pounds I’ll end up with on final install, but I’m not using a torque wrench for this step. You will, by the way, feel the effects and wisdom of anti-seize in this process. Do that a couple more times. This has seated the surfaces. It’s a sort of “truing,” but it’s really more of a mating. It’s facing down slight surface imperfections on the receiver, mostly.

Now, get your torque wrench and install the wrench head. Tighten the barrel nut until the wrench clicks. Only do this once. Don’t continue to repeat the clicks (you’re adding additional torque when you do).

Now. Here’s the trick: The gas tube has to be dead-centered in whatever it passes through. That might be one of the scallops on the barrel nut or a passage in the barrel nut, and, ultimately, the passage through the upper receiver. On some systems, there’s no association between gas tube and barrel nut. Those are bliss.

I’ve never seen a good torque wrench that would “add” torque if you repeat the clicks.

At any rate, feel free to weigh in with your own experience if you’ve done this before.


Comments

  1. On August 5, 2019 at 11:44 pm, James said:

    For the last two/three years I’ve been using the Geissele Reaction Rod in lieu of an upper receiver fixture. This allows me to barrel uppers that aren’t milspec like side charging uppers.

    I’ve used both the upper fixtures and the reaction rod; the latter is so much easier especially when mounting/removing muzzle devices.

  2. On August 6, 2019 at 10:17 pm, MTHead said:

    First thing I do is check the barrel/ upper fit. It should be very snug, but should go all the way up against the star chamber stop. Depending on which barrel nut your using will tell you which way from here. If your using mil. spec. or one that the gas tube has to fit through. I tighten the barrel nut down by HAND first. twist it down hard. Then look at the hole alignment through to the upper. Hopefully it will need just a third to half a turn to finish alignment. Tighten it to that alignment and that’s it. If you go passed you will have to keep going to the next slot. which might be tough to get to. Also, at that point your crushing the aluminum of the upper into the star chamber stop. and restricting headspace. And once it’s crushed, it’s crushed. It’s not going to spring back.
    Don’t worry about the torque spec as the gas tube will keep it from coming lose.
    If you have a barrel nut that doesn’t capture the gas tube. go all manufactures specs.
    Another problem I’ve seen a lot is the gas block. Where the block meets the journal step in the barrel. If your not going to use the front handguard plate. On a mil-spec barrel. You still need to leave a .030 gap. If you don’t your gas block is closing off half your gas.( It’s the major cause of short stroking in most builds.) And a lot of manufactures are leaving that gap out for freefloat tubes. it’s something that trips a lot of people up. Hope it helps!

  3. On August 7, 2019 at 12:11 pm, ROFuher said:

    Anti seize compounds incorporating copper probably are the wisest choice with an aluminum component in the mix.

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This article is filed under the category(s) AR-15s and was published August 5th, 2019 by Herschel Smith.

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