While installing the barrel nut I cannot get a slot to line up with the gas tube hole. At 30 ft.lbs. it just barely covers the hole enough where the tube will not go in straight. But I cannot tighten the nut enough for it to open up- I can muscle it to 80 ft.lbs but it will not open it all the way up. Any suggestions?
Standard GI barrel nut or something else? In either case it’s best to try a different nut or a different receiver. However, I’ve had the same experience of being just past alignment with GI barrel nuts and I still always manage to get there at the high end of the torque spec with tightening and loosening (not all that excessively either).
Key things that might help: Lube, don’t be shy with it. Liberally apply it to the threads of the upper and the forward facing side of the barrel extension flange. Also, inside on the threads of the barrel nut is fine too, but it should be nowhere else.
I personally use a high moly content paste and not the the AeroShell stuff. This might make a difference in things always working out for me. I’m not saying you should use it, but it’s what I use (TS-70 moly paste).
Also, a good barrel nut wrench can make a hell of a difference in reaching alignment at or near the max torque value. As far as I’m concerned, the barrel wrenches from 2UniqueLLC are the only ones I care to use on standard barrel nuts. And you need to secure the receiver in a solid vise setup. I use a Wilton Bullet machinist’s vise, mounted to a very solid bench bolted to the concreate floor. I also (usually) clamp the upper receiver in clamshells with a fin topped reaction rod (in my case I prefer the one from 2UniqueLLC) used inside the receiver as support, but this also locks the barrel extension solid with the receiver so that the barrel extension cannot twist in the receiver while the barrel nut is being tightened and loosened and tightened again. I do shim the fin on the 2UniqueLLC reaction rod with those clear plastic snare drum strainer strips so that there is no play inside the receiver so that the FSB stays in perfect alignment. That is if the FSB was in perfect alignment to begin with before tightening anything down.
See the below video for some idea as to the method.
Shims.
Read this and see if it helps you out](https://ar15barrels.com/data/barrel-nut-index.pdf)
Sometimes a different barrel nut will work, but shims will allow you to work with what you have.
^^ Get shims in your life and don’t look back.
So it looks like shims may be the way to go. However, I am assuming the barrel nut will need to be removed to add the shims? The barrel has a FSB installed so it would end up being a bigger job than I hoped for. Note- this is my first AR built from stripped receivers so I am new to this.
ETA: just read I can snip the shim to install without removing the FSB.
Yep, you got it…
If you have further issues, ask away. Chances are I’ve probably screwed it up before and learned a hard lesson that may save you some trouble.
Go to Bison Armory. Nice video on installing the shims.
I ordered the shims from them and watch the video. Although what they show seems counterintuitive to maintaining headspace- they show installing the shims between the barrel extension and receiver as opposed to the barrel nut and extension.
The receiver shims won’t affect headspace. I just like the idea of the barrel nut shims better so I went with those.
That won’t effect headspace. But, it’s how you have to use them if you cut the shims (which you have to to get around the indexing pin). They cannot be placed between the barrel nut and the barrel extension flange if they are cut, as the shim will bunch up on itself and become damaged. You can only stick them between the barrel nut and barrel extension flange if they are solid as they come.
Now something that can happen when using a cut shim between the receiver face and the barrel extension flange is the bolt carrier will seat ever so slightly forward. Usually it’s not enough to make any difference, but you could run into the issue of the buffer impacting the buffer retainer pin if things were already a bit too close to begin with.
^^ I forgot but that’s the reason I took off the FSB and didn’t cut it. It’s foolproof if left uncut.
Installing FSB without the gas tube? I guess it can be done. You don’t need to uninstall the FSB to undo the barrel nut for shims.
Different people have different methods. I do the following.
Lap the receiver shoulder for evenness.
Apply grease to receiver threads and barrel collar. Torque barrel nut to 35 ft-lbf, back up, and proceed to 40 ft-lbf. Keep going back and forth for every 5 ft-lbf step. You will see the nut progresses more steadily than going for the broke straight. Stop when you have alignment or reach 65 ft-lbf.
If no alignment at 65 ft-lbf, undo barrel completely and repeat process a couple more times. Still no alignment, I will lap the receiver shoulder a little more and repeat. It is going to line up, dollar to donut.
-TL
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I think you’re confused by my comment. You most certainly do have to remove the FSB in order to completely remove the barrel nut to shim between it and the barrel extension. IF you don’t want to cut the shim that is.
Sorry the comment on gas tube was to the op. He installed the FSB on the barrel. Then the barrel nut, trying to align. Guess the next step after alignment is to install the gas tube. I would do it differently. Gas tube to FSB to make assembly, barrel nut to align, then FSB/gas tube assembly to barrel. But it will work either way.
As for the shim. It goes between the receiver and barrel collar. Loose the barrel nut without taking it off the barrel (no need to remove FSB). Pull the barrel off the receiver. Put shim (no cut) under the barrel shim and reinstall barrel.
If the shim is between the barrel nut and barrel collar, then you are correct. The FSB will need to be uninstalled to remove the barrel nut completely. However, removing FSB shouldn’t be that hard.
-TL
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Being his first build, I doubt he has the necessary equipment for lapping the receiver face.
However, to the OP, you should be using grease on the threads of the receiver and barrel nut when you are torquing. If you were not, this alone may get you where you need to be…
I’ve not seen enough movement to matter though by torquing and backing off 3 times as a normally do when installing a barrel.
A lapping tool costs $30-$40.
The barrel nut has 18 tpi thread pitch, or 0.056" per turn. There 20 notches in the nut. The amount to lap away to for alignment is probably no more than 0.0015". That’s 1/3 of a paper thickness.
-TL
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I greased the threads and then torqued the nut and backed it off three times. I then proceeded to do it three more times up to 80 ft. lbs. to try to get it to index with no luck.
I’ve done six builds over the past four years. I had to use shims on two of them. Both used BCM uppers, one was a Spikes barrel, one was a Colt barrel. Both were way over 80 ft lbs to index the gas tube. In fact the Spikes was probably closer to 100. Then the Colt barrel was going the similar route. Maybe they would have been fine, but it just seemed wrong to go that high. Shims were the simple and easy solution, with both indexing as I recall between 30 to 40 ft lbs.
Sorry to hear that. Did you grease the barrel collar, where it interfaces with the underside of the barrel nut? If still not indexing well, shimming / lapping will be the option.
How do you fix the upper receiver when you torque the barrel nut? Is the ft-lbf figure what the torque wrench set up to? Because of the added length of the barrel nut wrench, the actual torque to the barrel nut will be higher, by as much as 20%. That’s why I usually stop at 65 ft-lbf indicated.
-TL
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Lapping tool $30
Lapping compound $30+ (Both OOS at Brownells)
Does he have a suitable drill? $100+
Does he have a suitable fixture to hold the upper for lapping?
It is a rabbit hole he doesn’t need to travel as shims are less than $10 and are also known to work. Plus, you don’t necessarily NEED to lap the face of the receiver when you build a rifle.