barrel index/alignment

Discussion in 'GATE AR-15 Forum' started by Bourbonator, May 7, 2014.

  1. Bourbonator

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    Apr 6, 2012
    I purchased a complete 16" upper from BCM just over a year ago, but due to life/scheduling, I just got the rifle out to zero the sights last week. Using Troy fixed sights, I needed 21 clicks right windage to get on target at 50m. From what I've read, this is out of spec. I can't attest to the straightness of the rail system, which is a Midwest Ind. T12G2, however, the barrel, at the end of the rail, is offset 1mm to the left inside the rail. It seems to me, that if the rail is indeed straight, the incorrect barrel indexing/alignment could be accountable for my extreme windage adjustment, at least in part. From what I can tell, the sights are level on the rail.
    This is my first rifle and AR, so I'm not very knowledgeable of what steps I should take to correct this issue. Should I take the gun to a smith to verify the issue? Contact BCM? Ignore it? What other precautions do you recommend before I take any action? Please keep in mind I currently live in an apartment with few tools and no workspace. Thanks for your time.
  2. Constructor

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    Dec 10, 2009
    Without having it in hand I would say the receiver face is out of square and making the barrel point off to one side.
    Installing the front sight on the forearm may not be the most stable or accurate place to mount it. Brownells sells a receiver squaring tool or if you are really lucky your local gunsmith may have one.

  3. R3508

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    Dec 9, 2005
    State of Jefferson USA
    I would give them a call and send it in.
  4. K511

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    Oct 13, 2015
    To answer the question directly, It's normal. Please understand the following, I had built several. They have a design flaw that hinders reliability and sight to barrel alignment. This would be the barrel nut effect. The reverse torsion on the barrel flange while in compression of a threaded flange nut causes this. Nobody has ever made a AR15/M16 w/the barrel true to the upper receiver -PERIOD. Some are more appealing then the extremes. Look at the upper receiver like a 2X4 of lumber from the rear sight end along the body line it will appear bent to where the barrel attaches. This is the real reliability problem, not whether chrome lined or not (although this really helps with dirty powders). The bolts usually bind on the barrel lugs as a result, causing the sticking (you wouldn't want a beefier bolt) that would lead to a jam. Heaver buffers do improve feed reliability by allowing more weight in motion (such as the DPMS double tungsten). When building a 50Beowulf, it was a problem. The bolt extractor claw had to be relieved to clear the barrel lugs before the bolt would lock in. Because the barrel w/lugs canted off center.

    The rear sight mount should ideally be close to the barrel nut or on the handguard rails that are mounted w/a handguard cap. Without a end cap mount support the rail will not be centered with the barrel because they usually mount only to a barrel nut. I like to mount a 22MM red dot by the barrel nut (things don't get that close to me to need any red dot bigger than that).

    I trust that now you understand your gun would be considered normal because there aren't any spec tolerances (per se) on barrel cant other than what you decide you can tolerate. No two uppers are exactly the same. Those barrels cant however they want and it can not be eliminated by this barrel nut mount design. It's a relative development.

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