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Thread: Installing USGI barrel nut to specific torque/shaving any impeding teeth: problem?

  1. #11
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    Not sure where or who told you that, but it is wrong. Per the Technical Manual;

    Torque barrel nut assembly (19) again to 30 ft-lb (40.5 N-m) while maintaining sight alignment. Barrel
    nut assembly may be tightened beyond 30 ft-lb (40.5 N-m) to align barrel nut assembly serrations for
    proper gas tube clearance. Never loosen barrel nut assembly to align for gas tube clearance.


    Quote Originally Posted by Saginaw79 View Post
    I was told to tighten it down three times(tight back off, tight back off, tight) and if they don't align, you slightly back off it until you get the clearance for the gas tube. Since then I haven't even built mine just buy uppers already assembled for the most pat. Now I will seek other knowledge before I do it



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  2. #12
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    Quote Originally Posted by Saginaw79 View Post
    I was told to tighten it down three times(tight back off, tight back off, tight) and if they don't align, you slightly back off it until you get the clearance for the gas tube. Since then I haven't even built mine just buy uppers already assembled for the most pat. Now I will seek other knowledge before I do it

    Who told you to do it that way? Have been messing with the M16/AR platform since '85, have never heard of ANYONE stating to do that. You NEVER back off a torqued fastener.

  3. #13
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    Quote Originally Posted by Saginaw79 View Post
    I was told to tighten it down three times(tight back off, tight back off, tight) and if they don't align, you slightly back off it until you get the clearance for the gas tube. Since then I haven't even built mine just buy uppers already assembled for the most pat. Now I will seek other knowledge before I do it
    If you've ever tried it, you'd instantly see why it's not a good idea.
    "I never learned from a man who agreed with me." Robert A. Heinlein

  4. #14
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    Thanks for the replies, everyone.

    As for backing it off, I appreciate the advice, but like everyone else here has said, it's a no-no for me. I'd also prefer to avoid shims if I can. I don't like adding parts where I don't need to, especially something that small/thin.

    IG: I've read in a few places that 40 lb-ft is the way to go for accuracy, high enough above the 30 lb-ft minimum threshold to make sure the nut is stuck on there and won't loosen, but still low enough to avoid the loss of accuracy that comes with high-torque. Grant mentions it here, Brownells also mentions things like that from their gunsmiths (can be seen in their assembly videos), and I've read the same at places like Snipershide and other precision-oriented forums. Not saying it's true, but I have seen it a lot of places and was just trying to see if I could achieve it with a standard USGI barrel nut. I wanted my cake and to eat it too. :-P

    That said, it doesn't seem like there are any real mechanical reasons for not doing this other than it's, well, not the right way to attach a barrel nut as per the technical manual? I can't see this screwing up the rifle or causing any problems, but you guys have more build experience than I do. Can you think of any reason to not do this (torque to 40 or so lb-ft, shave any offending teeth)?

    And yeah, you're right -- probably overthinking it.
    Last edited by DreadPirateMoyer; 01-07-14 at 02:03.

  5. #15
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    Well I am not going to enter into a pissing contest about who is right or wrong. But, I know I have had to tighten some that were surely not at that range and they were just fine. Most people cannot even shoot to the accuracy potential of their own weapons.



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  6. #16
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    I just installed some JP modular handguards, which uses the same tooth design as the standard AR barrel nuts. In the instructions for installation they wanted the barrel nut torqued from 50-80 ft. lbs. and stated that if a tooth on the barrel nut got in the way of this torque range that it could be shaved off without any issues. So I don't think shaving a tooth off of a standard barrel nut would be any problem.

  7. #17
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    I'm no expert but I just bought my first free float rail so I spent a lot of time researching how the various options install. I didn't see you specify which rail you were planning to use, but I believe a few of the rails that use the standard barrel nut require the teeth to be aligned properly in order for the rail to align with the receiver.

  8. #18
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    A couple of my thoughts....

    - IMO/E shaving of a tooth will 99.9% not have a negative effect.

    - I prefer lapping vs. shims if such an instance is required. It is my belief that lapping may also have a minute benefit of potentially truing the shoulder on the barrel opening of the receiver whereas a shim may have the slightest potential to have the opposite effect. Nitpicking but just my preference. I will also add that there is rarely times that this is ever necessary.

    - Working the threads on the receiver has a few benefits IMO and there is a difference in doing a standard 30 loosen, 30 loosen, 30 loosen before setting final torque vs the need to tighten and loosen in ascending ftlb increments to achieve a final alignment. But I am not going to get into it.

    - NEVER loosen anything to achieve final alignment. This is why barrels have loosened over time causing serious accuracy issues or why a crush washer and muzzle devices that need to be timed correctly come loose, because people back them off slightly if they go too far. Backing off a final torque value also applies to about anything outside of this particular topic also.

    - IME the lower 30 ftlbs range is what can cause accuracy issues over time as barrels have been known to come ever so slightly loose, even factory offerings from top name rifle makers.

    - I like to be around the 50-55 ftlbs range in a nut that does not require indexing.

    - Torque values are can be deceiving depending on how you set up the wrench on the armorer tool, dry or lubed part, accuracy of the tool itself.

  9. #19
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    Quote Originally Posted by TF82 View Post
    I'm no expert but I just bought my first free float rail so I spent a lot of time researching how the various options install. I didn't see you specify which rail you were planning to use, but I believe a few of the rails that use the standard barrel nut require the teeth to be aligned properly in order for the rail to align with the receiver.
    Exactly what I was thinking.

  10. #20
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    Quote Originally Posted by Surf View Post
    A couple of my thoughts....


    - I prefer lapping vs. shims if such an instance is required. It is my belief that lapping may also have a minute benefit of potentially truing the shoulder on the barrel opening of the receiver whereas a shim may have the slightest potential to have the opposite effect. Nitpicking but just my preference. I will also add that there is rarely times that this is ever necessary.
    Hi, I'm not trying to hijack this thread, but I saw lapping mentioned as a possible solution to barrel nut alignment issues. I've been looking at a lapping tool on Brownell's because I'm not happy with the torque range I'm getting with a NSR barrel nut. Either way I turn the nut the range is either too high or too low (Noveske barrel and upper).

    Would it make sense to lap the upper slightly to adjust the torque? From what I read above this sounds like a viable option. I've never liked the idea of a shim of any kind.

    Also, do you know of a place where I can read about the proper procedure for lapping the receiver, since I've never attempted to do this before? Any help would be appreciated thank you.

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