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Page AR-15 » Build It Yourself
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Posted: 1/24/2009 12:14:16 PM EDT
Love this forum, gave me a lot more confidence in approaching my first build.

I have a problem with indexing the barrel nut, though, and even though I have it sort-of set now, I'm not too happy with it. Wanted to see what everyone else thought.

30 ft-lbs of torque put the gas tube cutout just on the far side of the gas tube hole in the upper. I tried cranking past, onto the next cutout, but stopped when it became obvious that would involve a sledgehammer to apply enough force to get it over - well, I'm exaggerating, but it would have been way more than the 80-ft lb limit I've read.

Currently the nut is on there with barely 30 ft-lbs, but somewhat off center - there IS some pressure applied to the side of the gas tube, and this is causing the whole gas tube to be just the slightest bit angled inside the upper receiver. I was able to bend it a touch so it seems like the gas key slides over it easily enough, but that seemed like a bit of a kludge solution.

What solution do you use if you are right on the border as far as barrel nut indexing? Is there any way to shim the barrel nut?

Or is bending the gas tube a bit to make everything fit an acceptable solution? (Obviously it is NOT bent enough to crimp it anywhere)

Teflon tape maybe?

For info: this is a FN M-4 style upper with an olympic arms SS barrel and a stock barrel nut that came with the oly barrel. Small bit of high-temp moly grease used on threads.

It will go out to the range today for first test-fire.

Link Posted: 1/24/2009 12:16:50 PM EDT
First thing to try is back off and tighten again. Repeat as needed; you should get closer to where you want to be as you do that.
Link Posted: 1/24/2009 5:27:04 PM EDT
Be careful of putting to much torque on the barrel nut if you are using barrel vise blocks. I had an Olympic barrel extension start to unscrew while tightening the nut at less than 80 ft lbs once.
Link Posted: 1/25/2009 10:22:45 AM EDT
First thing to try is back off and tighten again. Repeat as needed; you should get closer to where you want to be as you do that.


Make sure you have greased the threads generously as well.  After torquing/loosening 5 or 6 times, you should be able to wrench it on to the next slot without exceeding 80ft-lbs.

I trust you are using a recevier clamp rather than barrel vise jaw blocks???  I hope so.

Link Posted: 1/25/2009 11:45:41 AM EDT
Well, a short break-in / test fire at the range went poorly. Several failure to fires - the bolt would not fully close due to the slight cant in the gas tube.

Took it home, back on the vice (yes, response to prev post- receiver block not barrel block) and after several cycles of loosening & tightening managed to get it to index properly in the next cutout. It may be a tad over 80 ft-lbs, but no sledge hammer was involved thankfully.

Something else occurred to me though - what about cutting off the offending tooth? Dremel it down to flush or something? Only one tooth missing off barrel nut shouldn't cause any difficulties with future removals... can anyone think of why that's a bad idea (aside from the cosmetics)

Link Posted: 1/25/2009 1:06:52 PM EDT
The gas tube prevents the nut from moving very far if it starts to loosen.  With no tooth, it can turn too far.

That's my answer and I'm sticking to it.
Link Posted: 1/25/2009 1:26:27 PM EDT
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