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Thread: "F" FSB & Troy Flip-up

  1. #11
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    ETA: Post Deleted, my browser refreshed but failed to show replies and answers to your question until I submitted my reply.

  2. #12
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    Why the F marked FSB is taller

    The top of a M1913 receiver is thicker than a standard receiver, which increases the height above bore of the rear sight. The front sight has to be raised to match.

  3. #13
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    The hoop on an A1/A2 should add rigidity compared to a flat-top, so it makes sense that a flat-top upper would be thicker.

    Why wouldn't the difference be taken up by items to be mounted on the flat-top? Why mess with the front sight?

    Maybe I'm deluded but it seems this system has remained pretty consistent over the years. Some changes, such as rifling twist, are pretty easy to comprehend. This front sight business puzzles me -- if there's a difference that makes a difference, there ought to be a good reason for it.

  4. #14
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    It's not a flattop thing, it's a carbine barrel sight radius thing.

    Even earlier carbines built on A1 type receivers had taller front sights when the sight was moved back to the carbine location.

    What front sight post does an M16A4 use?
    Randall Rausch
    AR15 Barrel Guru

  5. #15
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    I have Tory BUIS on my Sabre mid-length and I was able to get it zeroed at 50 yards without any issues. I built my upper and installed the barrel to a CMT upper receiver. Could this issue also be caused by an out of spec upper receiver rail?

    If your FSB is indeed shorter than a F marked FSB should be I think the easiest thing to do would be to get a longer front sight post.

    Keep us posted on how it goes.

  6. #16
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    Quote Originally Posted by AR15barrels View Post
    It's not a flattop thing, it's a carbine barrel sight radius thing.

    Even earlier carbines built on A1 type receivers had taller front sights when the sight was moved back to the carbine location.

    What front sight post does an M16A4 use?
    I turned my A2 into an A4 by adding a Colt flat top upper. Still has the standard FSB and it zero'd perfectly with the front sight flush, rear bottomed out on 6/3.
    Instructor: Sniper, Carbine, Handgun, Shotgun
    Armorer: Glock, Colt AR15/M16
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  7. #17
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    Quote Originally Posted by AR15barrels View Post
    It's not a flattop thing, it's a carbine barrel sight radius thing.

    Even earlier carbines built on A1 type receivers had taller front sights when the sight was moved back to the carbine location.

    What front sight post does an M16A4 use?
    Are we talking sight posts or sight bases?

    I had a Colt 20" flat top HBAR that came with an F marked base. And I also have an 11.5" Colt A1 upper (with m4 feedramps ) with an unmarked FSB.

    I don't know the answer, but these are two examples that I've seen first hand.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

  8. #18
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    Quote Originally Posted by M4arc View Post
    I have a Sabre mid-length upper with a Troy BUIS. My front sight base is marked with an "F" but the front sight post is cranked up pretty high. I'm trying to figure out what is wrong here; is it the front sight base or is it the Troy BUIS?

    Did Sabre use a standard FSB and just mark it with an F or does Troy build their BUIS to match the original height of an A2 upper?
    You can always order the taller front sight from Brownells if you can't get your upper to zero. I had to do this with a Bushmaster upper, but it had the standard height front sight base.
    "Pick up a rifle and you change instantly from a subject to a citizen."--- Jeff Cooper

  9. #19
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    Quote Originally Posted by K9-Bob View Post
    You can always order the taller front sight from Brownells if you can't get your upper to zero. I had to do this with a Bushmaster upper, but it had the standard height front sight base.
    The reason I resist doing this is that the higher a front sight sits in the FSB, the more prone it is to being damaged.

    I've had to replace dinged front sight posts before, so I feel warmer when they sit lower in the base.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

  10. #20
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    Quote Originally Posted by demigod View Post
    The reason I resist doing this is that the higher a front sight sits in the FSB, the more prone it is to being damaged.

    I've had to replace dinged front sight posts before, so I feel warmer when they sit lower in the base.
    You must not realize that an extended post in a standard A2 FSB sits EXACTLY the same height as a standard post in an F FSB does.
    The height of the ears in relation to the post is the same in both cases.

    The same forging is used for both A2 and F front sight bases.
    It's only the height of the shelf that's different.
    An F front sight base has shorter "ears" than an A2 does.
    Randall Rausch
    AR15 Barrel Guru

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