Problem with barrel nut

[Waylander]

^^ I forgot but that’s the reason I took off the FSB and didn’t cut it. It’s foolproof if left uncut.

[tangolima]

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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[Waylander]

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.

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.

[tangolima]

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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[HKGuns]

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.

[tangolima]

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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[flenna]

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.

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.

[bobcatdriver]

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.

[tangolima]

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.

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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[HKGuns]

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

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.