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Thread: Interesting issue with my KAC URX 3.1 build

  1. #11
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    Is there an upper receiver that isn't affected by this? Meaning, are these tolerance issue more prevalent with a colt upper vs. a noveske upper (Just throwing those out there not being specific)?


    Quote Originally Posted by mtdawg169 View Post
    This. The issue arises due to tolerances in the upper receiver threads and the urx threads. You may have to try multiple upper receivers to get better alignment, but as IG noted, 15 clicks is within acceptable limits.

  2. #12
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by davidjinks View Post
    Is there an upper receiver that isn't affected by this? Meaning, are these tolerance issue more prevalent with a colt upper vs. a noveske upper (Just throwing those out there not being specific)?
    Iirc, vltor murs are known for their tolerances.
    "I never learned from a man who agreed with me." Robert A. Heinlein

  3. #13
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by Koshinn View Post
    Iirc, vltor murs are known for their tolerances.
    I've had very good luck with MUR uppers. Two upper builds with URX rails and minimal windage adjustments needed for the irons. I had them installed by professional armorers though. I'd have to be installing alot of urx rails to justify the cost of setting up to do it properly, as it involves alot more than just buying the wrench, IMHO. Ayan at boltcarrier.com does a very good job.

    David, who assembled your upper?
    Last edited by mtdawg169; 06-13-13 at 13:38.

  4. #14
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by Mrick View Post
    I took my newly assembled upper out to zero over the weekend and quickly realized something was not right. I needed upwards of 15 + clicks of right windage to zero which while not extremely excessive was too much for my taste.

    My first thought was that the upper and rail were out of alignment but this was not the case. I used a rail vice block while assembling with loctite and upon inspection the rail is still inline with the upper. I mounted a carry handle across to verify and it fits perfectly so it's not the rail alignment to the upper that's the issue.

    I then started looking at the gas block and it's relationship to the rail. It appears the gas block looks to be closer to the left side of the rail and possibly ever so slightly rotated to the left as well. I've included a
    pic of the gas block looking down the barrel.

    My question is...what part is out of spec? The upper? The URX 3.1? It's driving me a little crazy trying to see just what is out of alignment!

    Rob

    Parts:
    BCM Middie 16" Govt with BCM low pro gas block
    Aero Precision upper receiver



    Shot of barrel in rail- Note the left to barrel clearance looks smaller than the right to barrel near the gas block

    OP, your situation is a little different than what I've seen in the past. Usually, the gas block sits more to the left (when viewed from the muzzle). Odd.

  5. #15
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    Quote Originally Posted by mtdawg169 View Post
    OP, your situation is a little different than what I've seen in the past. Usually, the gas block sits more to the left (when viewed from the muzzle). Odd.


    This has been my experience as well.
    "Not every thing on Earth requires an aftermarket upgrade." demigod/markm

  6. #16
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by scottryan View Post
    This has been my experience as well.
    I'm wondering how the upper and rail were secured during installation?

  7. #17
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    Quote Originally Posted by mtdawg169 View Post
    I'm wondering how the upper and rail were secured during installation?
    The PRI rail vice block was used.

  8. #18
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    Quote Originally Posted by Mrick View Post
    The PRI rail vice block was used.
    IMHO the PRI rail block is not an adequate solution to mount a URX rail correctly... Everything - the rail, the upper, the block - flexes if you apply more than 40-45ft/bl of torque to the barrel nut.
    Scout Rider for the Mongol Hordes

  9. #19
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by caporider View Post
    IMHO the PRI rail block is not an adequate solution to mount a URX rail correctly... Everything - the rail, the upper, the block - flexes if you apply more than 40-45ft/bl of torque to the barrel nut.
    I would tend to agree. I know alot of guys have made it work, but the KAC vise block or a set of custom vise jaws like Ayan uses are really needed to keep everything straight during installation.

  10. #20
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    You need real old school american made vice for this with atleast a 6" jaw. An old school one where the jaws are straight and no play, using the KAC fixture is good.

    I use an old Columbian vice that belonged to my great grandfather that never really got used for anything and it is still like new and straight.

    A mill vice with custom jaws like what Ayan uses is also good.

    Harbor Freight / Lowes / Home Depot china shit isn't going to cut it for this.

    For a URX install, everything needs to be perfect. You need a high end barrel, a name brand flattop receiver, a real vice, and the KAC tools.
    Last edited by scottryan; 06-13-13 at 17:50.
    "Not every thing on Earth requires an aftermarket upgrade." demigod/markm

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