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Thread: Interesting issue with my KAC URX 3.1 build

  1. #1
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    Interesting issue with my KAC URX 3.1 build

    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

    Last edited by Mrick; 06-12-13 at 21:55. Reason: Added another pic

  2. #2
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    I'm not an expert, but just a thought; since you aligned the rails with an alignment block, the receiver and rail should be aligned. That can only mean the barrel isn't seated in the receiver quite right or something is bent. I guess this is why the folks who depend on their iron sights go for the pinned FSP. They don't have to worry about barrel-to-front-sight alignment.

    In the case of a FF hand guard and rail-mounted front iron sights, all three parts, receiver (rear sight), barrel and rail (front sight) should be in alignment with each other or you'll have to adjust for it in the sights.

  3. #3
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    Do you have a vise and receiver block? I'm not sure how to check barrel to receiver alignment, but it could be that your barrel isn't fully seated into the receiver. Maybe hang your receiver vertically and use a plumb line to check whether the barrel or rail is crooked?
    Last edited by crosseyedshooter; 06-12-13 at 22:54.

  4. #4
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    Quote Originally Posted by crosseyedshooter View Post
    Do you have a vise and receiver block? I'm not sure how to check barrel to receiver alignment, but it could be that your barrel isn't fully seated into the receiver. Maybe hang your receiver vertically and use a plumb line to check whether the barrel or rail is crooked?
    I do have a vise and receiver block. I can't see how the barrel isn't fully seated as looking at the chamber all is good and there is the recommended 50 ft lbs of torque applied to the KAC Nut which was loctited and progressively achieved during the install.

  5. #5
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    15 clicks isn't excessive, regardless of your taste.



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  6. #6
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    If you want to see what's out of alignment, use a carpenter's square against the upper's rail. Just eyeballing it I can see the gas block looks to the right of the barrel nut. But can't tell if it's the pic not being square or the upper itself.

  7. #7
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    This is a known issue with KAC URX rails. You must test fit the parts before assembling.

    You might have to try several uppers before you get alignment.
    Last edited by scottryan; 06-13-13 at 10:05.
    "Not every thing on Earth requires an aftermarket upgrade." demigod/markm

  8. #8
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    Quote Originally Posted by scottryan View Post
    This is a known issue with KAC URX rails. You must test fit the parts before assembling.

    You might have to try several uppers before you get alignment.
    That is probably true. I may just go with another rail setup.

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

    Quote Originally Posted by scottryan View Post
    This is a known issue with KAC URX rails. You must test fit the parts before assembling.

    You might have to try several uppers before you get alignment.
    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.

  10. #10
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    This is some interesting information. I did not know this.

    I've been going through the same issue with an upper I had built with a URX 3.0 rail. My barrel was way more off than the OPs and my cut FSB was making contact with the rail while shooting it. At 100 yards I stopped adjusting after 18 clicks left.


    Quote Originally Posted by scottryan View Post
    This is a known issue with KAC URX rails. You must test fit the parts before assembling.

    You might have to try several uppers before you get alignment.

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