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Thread: Sighting in new BCM 14.5 middy with LaRue BUIS...is this a problem?

  1. #1
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    Sighting in new BCM 14.5 middy with LaRue BUIS...is this a problem?

    Hey guys. I rarely post here as I'm basically a lurker taking in as much information as I can and learning as I go along, but I ran into an unusual situation today at the range and I'm curious to know if this is something to be concerned with or not.

    Before I get into the issue...A while back I purchased a Magpul MBUS for my LE6920 as I had removed the carry handle in order to mount a RDS. Zero the MBUS at 50yds with the base of the front sight post flush with the top of the FSB. A couple windage adjustments and I was dead on without having to make any elevation changes. GTG.

    A week ago...I got in a new BCM 14.5 mid-length upper. Now what I don't want to do, is move my red dot and MBUS from the Colt over to BCM since the 6920 is zero'd.

    A year or so ago, I picked up a LaRue BUIS that I never really used...so I figured I would mount the LaRue to the BCM, zero it...and then whenever I can pickup another lower, I'll have another AR build to play with.

    Here is the LaRue BUIS I'm referring too...


    Ok...start out with base of the front post flush with the FSB, rear aperture (small) centered. Take 5 shots at 50 yds. In the picture you can see the first group at the top right...then you can see the adjustments as I walked in my rounds to the center of the target.




    Now...this is what has me at pause...



    My question is...is it normal for the front sight post to be this elevated after a 50 yd zero depending on what rear BUIS may be used? I was under the impression that rear sights be they folding or not...were supposed to be pretty close to the same height off the upper receiver...but I may be wrong. IDK.

    Remember...the front post on my Colt needed no adjustment at all. It's still flush with the FSB and it's dead on at that same range.

    What I may do, is mount the carry handle off the Colt onto the BCM and re-zero to see if there is a change.

  2. #2
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    No, that is not normal.
    Something is off somewhere.
    Easy answer is to get a taller front sight post.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  3. #3
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    Quote Originally Posted by Failure2Stop View Post
    No, that is not normal.
    Something is off somewhere.
    Easy answer is to get a taller front sight post.
    I thought of that as well, but if it needs a taller front sight post, then I would think BCM would make that the standard.

    I think I'll test out the carry handle and maybe even pull the MBUS off the Colt if I have too. If I still have the same problem, then I know it's the front site. If not...then the LaRue is screwed up.
    Last edited by JohnnyNumbers; 04-22-12 at 18:11.

  4. #4
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    Looks like the sight post is above the ears though.
    I have the same LaRue buis on one of my rifles and several other brands as well (DD, Troy). None have the sight post that high, most are flush w/ the top of the FSB or lower. Try the carry handle, that should narrow it down to the sight or the upper.
    "We prepare, so we don't end up at the superdome"- unknown

    "IMHO, if you wanted to shoot crap ammo, you should have bought a crap upper. It makes baby Jesus cry when he sees crap ammo put through a nice upper."- C4IGrant

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    Quote Originally Posted by sewvacman View Post
    Looks like the sight post is above the ears though.
    I have the same LaRue buis on one of my rifles and several other brands as well (DD, Troy). None have the sight post that high, most are flush w/ the top of the FSB or lower. Try the carry handle, that should narrow it down to the sight or the upper.
    Yeah that's where the sight post ended up after I got it sighted in. I started off with the front post flush with the FSB.

    When my first group was like 6" high at 50 yards... I knew something wasn't right.

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    Quote Originally Posted by JohnnyNumbers View Post
    I thought of that as well, but if it needs a taller front sight post, then I would think BCM would make that the standard.

    I think I'll test out the carry handle and maybe even pull the MBUS off the Colt if I have too. If I still have the same problem, then I know it's the front site. If not...then the LaRue is screwed up.
    Is the FSB "F" marked?
    Usually BCMs are good to go, but sometimes things fall through the cracks even with good companies.

    I also misspoke, the "easiest" answer is to swap rear sights if you have one available.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  7. #7
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    Quote Originally Posted by Failure2Stop View Post
    Is the FSB "F" marked?
    Usually BCMs are good to go, but sometimes things fall through the cracks even with good companies.

    I also misspoke, the "easiest" answer is to swap rear sights if you have one available.
    Yes...the FSB is most definitely "F" marked.

    I'll get back to the range as soon as I can.
    Last edited by JohnnyNumbers; 04-22-12 at 18:48.

  8. #8
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    try another rear sight,my troy lets my front post set two clicks lower than my LMT or GGG rear sight.

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    Update...

    Ok...back to the range today with carry handle off my Colt mounted on the BCM upper.


    FSP reset to flush and the rear sight set to 6/3 and centered. Range is 50 yards...




    Now from what I have read using M193 w/14.5" barrel and hitting about an inch high at 50 yards...should put me right on at 100yds. At least thats what I thought I read somewhere on this forum... I may be wrong.

    Move to the 100 yard range...thinking I should be hitting around the general vicinity of the 10x with my grouping, granted my fundamentals are in check.

    Fire a 5 shot group...



    That first group is roughly 5" high from center.

    Make a 1 click FSP adjustment and lower my POA to where I have it marked on the target (at least that's where it appeared looking thru the aperture at that distance and my eyes aren't as good as they used to be) which still has me printing 2" high from center of the 10x.

    I could make further adjustments raising the FSP to bring my POI lower...but here we are getting back to raising my FSP up to a point to where it's as high or higher than the ears of the front sight base, which to me isn't right...and the whole purpose of this thread.

    Here is where the FSP is set as of now...



    So again I ask...do you guys think this a unusual? I could just remember that I'm hitting 3" high at 100 yards or just go ahead and see about getting a taller FSP. It just seems to me that if a taller post is needed...BCM should make that a standard.
    Last edited by JohnnyNumbers; 04-24-12 at 16:58.

  10. #10
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    Why not set it at 300 yards like that carry handle is designed for? Zero at 25 and you should be about 6 high at 100 and zero at 300. Once set at 25 verify at 300 and fine tune your zero.

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