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Thread: Interesting issue with my KAC URX 3.1 build

  1. #31
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by Mrick View Post
    I'd just like to take a second to thank Ayan for contacting me and offering to help out with the upper. He is truly a class act and I'll surely be doing business with him going forward.

    Rob
    Ayan is a hell of a guy!

  2. #32
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    Quote Originally Posted by CharlieMike View Post
    Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.

    The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.

    I think I have a fix for it. We'll see in the morning.

    -ayan
    3 factors are at play:
    Relationship of the barrel to the rail
    Relationship of the rail to the upper receiver
    Relationship of the barrel to the upper receiver

    There are factors that effect each of those.

    Tolerance stacking in a few minor areas, or a significant deviation in one single area can result in the barrel not being perfectly centered in a floating handguard; without having any effect on performance.

    These issues are pretty much never a factor in a non-ff handguard because deviation is masked by connecting the handguard to the barrel. A system that never before cared if the barrel extension perfectly flushed out on the front of the upper receiver is now expected to demonstrate virtually no deviation there. The system never functionally cared if the top of the upper receiver was perfectly parallel to the barrel. All of these legacy aspects can combine with application of torque to cause the barrel to not sit perfectly centered in an otherwise perfectly functional gun with quality parts all around.

    From Ayan's description, it sounds like the receiver rail and URX rail don't quite agree on what "parallel to the barrel" is.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  3. #33
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    Quote Originally Posted by Mrick View Post
    That is probably true. I may just go with another rail setup.
    Let me know when you do, I'll take that "terrible out-of-spec" URX 3.1 off your hands.
    US Army Infantry 2008-2012
    LEO 2012 - Current

  4. #34
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    I'm pretty sure you meant this as sarcasm, however some people may get the wrong impression of your intent. I don't believe anything is out of spec in regards to the rail, upper or the whole enchilada.


    Quote Originally Posted by ZoneOne View Post
    Let me know when you do, I'll take that "terrible out-of-spec" URX 3.1 off your hands.

  5. #35
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    Well mine came back a couple days ago, Rainier had it turned around in like 2 days. I wont be home 'till next week, but had some pics sent to me and it looks perfectly aligned. Colt upper, Compass lake Krieger barrel, and BCM BCG so it was quality parts.

  6. #36
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    I'm going to say it again.. machine the end of the URX rail.
    My recent SBR build had the same issue, just not as extreme.
    Everything sits perfectly centered now after the back of the rail was cleaned up. I can't post pics because the parts are getting cerakoted right now.

  7. #37
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    Re: Interesting issue with my KAC URX 3.1 build

    Quote Originally Posted by E-man930 View Post
    I'm going to say it again.. machine the end of the URX rail.
    My recent SBR build had the same issue, just not as extreme.
    Everything sits perfectly centered now after the back of the rail was cleaned up. I can't post pics because the parts are getting cerakoted right now.
    Who machined the rail for you? Did you take enough off to flush it up with the upper receiver and "time" it?

  8. #38
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    Quote Originally Posted by mtdawg169 View Post
    Who machined the rail for you? Did you take enough off to flush it up with the upper receiver and "time" it?
    I did, and yes of course it is timed with the upper receiver for a flush fit.

  9. #39
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    I got an AAC MPW today with the URX 3.1 factory installed and it's noticeably crooked as well, but to the opposite side of the OP's rifle. I haven't shot it yet so I don't know how many clicks it would take to zero the irons. What's the maximum number of clicks from center that would be considered acceptable?

    My SR15 IWS Mod. 1 upper has the same rail, but I cannot detect any crookedness by eyeballing it. In addition, the rail sits flush to the upper receiver, unlike the MPW which leaves a significant gap.
    Last edited by jumbopanda; 06-28-13 at 21:40.

  10. #40
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    Interesting issue with my KAC URX 3.1 build

    An update on my URX upper. Ayan @ boltcarrier.com took the time to hand fit 3 different upper receivers I sent him and found one that mated up perfectly with a new rail as the existing one had been beaten up from wrenching it apart several times. It was an AO precision upper with a rectangular forge marking for anyone whose curious. I received the perfectly installed railed upper a few days back and will hopefully get a chance to run some rounds through it soon. Many thanks to Ayan who will get all of my AR related business going forward.

    Rob

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