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Posted: 3/6/2014 10:25:21 AM EDT
[#1]
I've only built one AR upper so far and I had the exact same issue.
I torqued the nut to 35ftlbs then backed it off 3 times. On the final torque I could get the gas tube to fit snugly but the rail was out of whack noticeably. I tried torquing to the max 80ftlbs and it did not get anywhere near the second notch. So I ended up backing the nut off a smidge to probably about 26-28ftlbs to get the gas tube and rail lined up properly. Whether or not that's the correct thing to do I am not sure but it is what I did. |
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Posted: 3/6/2014 10:50:40 AM EDT
[#2]
Make sure you use grease on the threads. It helps to get your torque in spec while aligning the nut.
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Posted: 3/6/2014 11:52:59 AM EDT
[#3]
Thanks. I did grease the threads, I think I'm going to stick with it how it is now which is slightly under 30 lbs of torque. I've read about people grinding off the tooth that is in the way, but that just doesn't seem right.
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Posted: 3/6/2014 1:56:06 PM EDT
[#4]
There are shims for this issue, but I 've never had to use 'em. BFI was going to fix me up with some back around 2010, but I switched barrels to a Colt and bypassed the whole thing. I suspect that the Bushie barrel may have had an out of spec flange. I would NEVER grind off a tooth..just never !
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Posted: 3/6/2014 2:05:04 PM EDT
[#5]
Not a guru here but I've heard you can take her back to about 25 lbs if necessary for alignment.
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Posted: 4/9/2014 6:11:44 AM EDT
[#6]
I've run into this problem many times over the years, with some big-name mfg's too.
The solution is order another barrel nut (they're not expensive) or use shims. If it's a barrel nut for a rail, call the rail manufacturer and explain the issue. LaRue sent me a barrel nut free when I had this problem on a Colt build. |
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Posted: 4/9/2014 10:57:16 AM EDT
[#7]
One final suggestion. Take some fine grit sandpaper and work the front of the receiver in a figure 8 on a flat surface. You won't need many passes to remove A bit of material.
If you want something more precise for the job, Brownells and midway both sell a tool to lap the receiver . lapping tool Same idea but a bit more precise. |
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Posted: 4/9/2014 11:20:04 AM EDT
[#8]
Thanks for the suggestion. I worry about keeping the face square with lapping. Is that not an issue?
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Posted: 4/9/2014 11:27:12 AM EDT
[#9]
The lapping tool guides itself square.
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Posted: 4/23/2014 3:49:53 PM EDT
[#10]
Lube the threads and the barrel ring/flange. Slowly tighten and loosen the barrel nut until the barrel nut notch or bore looks fairly close to the receiver gas tube bore like this: barrel nut notch/bore>oo<receiver gas tube bore. Remove the nut, clean areas of contact thoughly, reapply grease, tighten nut until the tube can be precisely centered.
Over time 30 lbs. of torque on greasy aluminum threads becomes 25 lbs. As for facing the receiver it moves the barrel back and may put the feed ramps slightly out of what should be a smooth alignment. Using barrel shims especially very thin ones is not advised. |
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Posted: 4/24/2014 3:55:16 PM EDT
[#11]
I feel best with my barrels torqued at 55 foot pounds. (DD Lite barrel nuts)
With USGI nuts, though, I don't worry too much if I have to go 30, or even slightly under. Keep in mind the torque rating for the KAC URX rails is 20 foot pounds, and they're using an aluminum barrel nut with threadlocker!!! Edit: USGI barrel nuts are cheap. You can pick up takeoff's on the EE for like $5. I usually keep a few extras. Much like crush washers to index your flash hider, it's good to keep a few to try if one seems way off... Try another barrel nut if you can---might be a lot simpler and cheaper than buying lapping kits, etc. |
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