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Thread: FN M16A2 FSB vs. A3/A4 FSB - How to tell the difference?

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    FN M16A2 FSB vs. A3/A4 FSB - How to tell the difference?

    What markings on the FN M16 FSB ID's A2 vs. A3/A4?

    Thanks in advance.

    Celt
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    What difference? I thought the F marked FSB's were needed due to the shorter barrels.

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    Quote Originally Posted by Celt View Post
    What markings on the FN M16 FSB ID's A2 vs. A3/A4?

    Thanks in advance.

    Celt

    Why do you need to know this?

    What do you mean by ID? The F marking or the forge code?
    "Not every thing on Earth requires an aftermarket upgrade." demigod/markm

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    Quote Originally Posted by mack7.62 View Post
    What difference? I thought the F marked FSB's were needed due to the shorter barrels.
    No. The F marked fsb has a shelf that is .040" higher than standard because the rear sight on a flat top upper sits .040" higher than an A2. Has nothing to do with being on a carbine.

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    Quote Originally Posted by Rifleman_04 View Post
    No. The F marked fsb has a shelf that is .040" higher than standard because the rear sight on a flat top upper sits .040" higher than an A2. Has nothing to do with being on a carbine.

    That is true but isn't complete information.

    All carbines, regardless of fixed handle or removable carry handle, have a taller overall front sight arrangement.

    Examples:

    A Colt 727 will have a standard front sight base, but a taller front sight post.

    A Colt flattop carbine will have a taller F marked front sight base, but a standard height post.

    A Colt SP1 carbine has a taller round post than an SP1 rifle or M16/M16A1.

    The M16, M16A1, M16A2, M16A3, M16A4, M4, and M4A1 all use the same height front sight post, for logistical reasons.

    There several different posts that have been used:

    Standard Round = M16, M16A1, SP1 Rifle
    Tall Round = SP1 Carbine, XM177/GAU-5 Series, 653, 639
    Standard Square = M16A2, M16A3, M16A4, M4, M4A1, LE6920, AR6721
    Tall Square = 700 series carbines, 9mm SMGs
    Short Tapered Square = some 11.5" flattops and export flattop carbines, some 933, 977, 979
    Last edited by scottryan; 07-31-17 at 13:30.
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    Thanks for the added info.

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    Quote Originally Posted by scottryan View Post
    That is true but isn't complete information.

    All carbines, regardless of fixed handle or removable carry handle, have a taller overall front sight arrangement.

    Examples:

    A Colt 727 will have a standard front sight base, but a taller front sight post.

    A Colt flattop carbine will have a taller F marked front sight base, but a standard height post.

    A Colt SP1 carbine has a taller round post than an SP1 rifle or M16/M16A1.

    The M16, M16A1, M16A2, M16A3, M16A4, M4, and M4A1 all use the same height front sight post, for logistical reasons.

    There several different posts that have been used:

    Standard Round = M16, M16A1, SP1 Rifle
    Tall Round = SP1 Carbine, XM177/GAU-5 Series, 653, 639
    Standard Square = M16A2, M16A3, M16A4, M4, M4A1, LE6920, AR6721
    Tall Square = 700 series carbines, 9mm SMGs
    Short Tapered Square = some 11.5" flattops and export flattop carbines, some 933, 977, 979
    What about the 6720 (not 6721) ... ??? I've read where Colt used the standard FSB but a taller sight post. I'm wondering why on that model they are different than say a 6920?
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