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Thread: laser bore sights for ar15

  1. #1
    Join Date
    6th July 2021
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    laser bore sights for ar15

    I recently finished my first AR build in which I tried to use the best parts possible - or at least the best I could find & afford... - (Daniel Defense 5.56 16" govt profile barrel, 14" DD free float Lite Rail, LRB flat top upper & lower, Magpul UBR, etc). I just mounted an Aimpoint PRO and a set of Magpul MBUS back up sights on the AR and wanted to bore sight it before going to the range. I had to do some pretty serious adjustment to get everything aligned for windage, moving my rear MBUS aperture alsmost as far to the left as possible. Is this normal or am I doing something wrong? Could my laser bore sight for ar15 be off? Is my cheek weld wrong? I would appreciate any input.

    I mounted the Aimpoint and MBUS on the top rail and chambered my SightMark .223 laser bore sight in the barrel. To align everything, I was sighting down the hall in my home so that I could get the longest possible sight distance - which is only about 35 feet - projecting the laser on the farthest wall I could see. Standing with my back flat against a wall, I adjusted the windage on the rear MBUS so that the front sight post was aligned with the bore sight's POI (I didn't worry with the elevation, which had the POI a couple of inches below the top of the front sight post at 35 feet, because I'll actually zero the rifle at 50 or 100 yards when I get to the range. But I did want to get it close overall so that it would at least be on the paper...). Then I did the same thing with the Aimpoint, getting that "apple on a fence post" sight picture you're looking for. When I was done, like I said earlier, the rear MBUS was adjusted nearly as far to the left as it could go. This gave me the proper sight picture to get the Aimpoint, MBUS, and bore sight all in a vertical alignment.

    I've been hunting for years, so I know how to hold a rifle, (even though this is my first AR) and the hold I had on the gun was comfortable for me. I also used the edge of a door frame on the wall I was projecting the laser on to make sure everything was aligned vertically, so I'm fairly confident in my results. I just would't have imagined that I would have to adjust everything this far off center. So is this "normal" or should I be concerned?

    I appreciate any input you may have -- Thanks!

  2. #2
    Join Date
    29th August 2017
    Posts
    381
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    You can check the bore sight by rotating it 180 degrees (a half turn) in the chamber. If the bore sight is true, the dot will still be at the same previously adjusted point of aim.

    Assuming the bore sight is true, the front sight base is likely canted to one side. This can occur if the index slot in the upper is off, or the front sight base is off with respect to the index pin.

    When the front sight base is canted enough to require that much rear sight compensation, the canting is usually visible. You can check by looking back at the front sight base and upper from the muzzle end, and comparing the front sight base profile against the upper receiver profile.

    -

  3. #3
    Join Date
    29th August 2017
    Posts
    381
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    I didn't notice any mention of a specific front sight, so I assumed a conventional AR15 front sight base.

    Is the front sight rail mounted?

    If so, then check that the rail is centered about the barrel and is not rotated with respect to the upper.

    -
    Last edited by megafiddle; 13th July 2021 at 18:17.


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