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Thread: M4 front sight blade height

  1. #1
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    M4 front sight blade height

    I've just completed my second AR build, an M forgery, BCM 16" M4 barrel and receiver, LMT bcg, BCM/Troy charging handel and RRA six position stock. Rear sight is a carry handle that a fellow gave me that he said came off a new Colt.

    Everything works fine....except....using the accepted procedure for the 50 yard zeroing method, I have to run my front sight post "all the way out", and I'm still about six inches high at 100 yds, using different "standard" loads....and the adjustable rear sight sitting all the way on the bottom.

    I've got no more adjustment.

    Bushmaster has a "taller" sight blade that is .040" taller. Should this work?

    By the way, my front sight post is already an "F" variety?

    In the Bushmaster online catalog, they mention that Colt carry handle sights are a bit different and might require the taller post.

    Is this correct? Any experience? Any advice?

    Anyone used the taller post and have it stick out "above the ears?"

    Thanks in advance.
    "If you think everything is going to be alright...then you don't understand the problem." - Gutpile Charlie

    "A man's got to know his limitations." - Harry Callahan

  2. #2
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    Bushmaster's taller post should work.

  3. #3
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    From what I understand, Colt came out with the F front sight assembly to work with the flat tops w/removalable carry handle. I'm assuming that all F marked front sight towers are now an industry standard (same height) & should work with any quality forged removable carry handle.

    First, make sure the carry handle you have is a quality make (check the forging mark) and not cheap chinese junk (it could be out of spec).

    Second, if the CH is good and you want to use it, by all means switch to the taller post.

    If your just going to replace the CH down the road with a BUIS, you may just want to do that now. Especially if your going with a flat top rail mounted optic. A genuie Colt CH should sell for close to the cost of a quaility BUIS.

    As far as BM goes. They probably offer a taller FS post as their flat top uppers come with standard height sight towers and not F marked. At least this it the case with my newly purchased BM 6.8 M4 upper.

    Good luck.
    Last edited by Dump1567; 05-29-10 at 10:30.

  4. #4
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    Dump1567, yes the handle/sight I just put on is a high quality, forged and marked item. The guy who gave it to me said it just took it off a new Colt. I put it on because the first one I had was cheap/out of spec/junk!

    This one is truly a quality one, fits good, aligns perfectly, etc....but Bushmaster catalog just happened to mention that the "Colt" handles were perhaps just a bit different? It definitely will not go low enough with the front sight post I have now. It sounds like the added .040" inch post might just be enough to get the height that I need.
    "If you think everything is going to be alright...then you don't understand the problem." - Gutpile Charlie

    "A man's got to know his limitations." - Harry Callahan

  5. #5
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    Quote Originally Posted by Gutpile Charlie View Post

    In the Bushmaster online catalog, they mention that Colt carry handle sights are a bit different and might require the taller post.
    Different then Bushmaster or then the industry standard?

    Bushmaster doesn’t make F marked FSB’s IIRC, so yes you would need taller front sight when using one on a BM.

    I was under the impression that Bushmaster carry handles were different then standard so they would work with their non F marked FSB.

  6. #6
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    BCM's F fsb and Colt carry handle should work beautifully together.. if not, i'd be investigating why not, and fixing it. having your post extend above the ears on your base is gay and unacceptable.

    i'd try a different carry handle... hopefully you can borrow one or a different BUIS from somebody and see if that helps.

    F-marked FSBs should have a 1.98" shelf in the post cutout. if you can get ahold of some calipers with ID bars, this'll probably be the best way to measure. otherwise, a machining scale (a ruler that goes to hundredths or more) will probably get you close enough to see if this is a problem, and can be had for $5

    if those check out, i'd probably go ahead and pull the barrel and lap the face of the receiver. if lapping doesn't immediately show unevenness, i'd reassemble and try again- and if it still wouldn't zero, i'd give BCM a call, 'cause pretty much all that's left is an out of spec top rail or misaligned barrel extension. both are very unlikely, and i suspect a new rear sight will fix it.

    that's me, though. maybe a taller post is as much effort as you're willing to give it.

  7. #7
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    Problem fixed!

    Ordered +.040 height front sight posts from Bushmaster, installed them and headed to the range.

    With the Colt carry handle/sight set as per the instructions for the Improved Battlesight Zero (IBSZ), the extended length front sight works perfectly....and doesn't extend beyond the "ears" on the front sight post.

    This was the correct fix for me. It allows me to use the Colt handle that I had and zeros perfectly.
    "If you think everything is going to be alright...then you don't understand the problem." - Gutpile Charlie

    "A man's got to know his limitations." - Harry Callahan

  8. #8
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    Glad your problem was fixed, but it still does noe make sense. If you have a BCM upper, and a real Colt carry handle, you should never need the taller FSP.

  9. #9
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    I suspect that his Colt carry handle is not a Colt but a Bushmaster.

  10. #10
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    Could you be using a slightly different sight picture than normal for zeroing maybe?

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