I  know I've seen this discussed before but can't remember. An "F" marked FSB must be used with a flattop upper receiver. Correct? Otherwise the front sight post will be up too high? But, a standard FSB works with an A2upper. Correct? And this is on a carbine using a removeable carry handle.

AFAIK,
a F-FSB is needed on ALL carbines; both A1/A2 and flat top.


ETA:
a couple of weeks ago someone explained it very well and also posted a simple pic...I can remember the user's name...

Frens is absolutely right.

While it's true that the flat top M4 and F base were introduced at thew same time, it doesn't have a darn thing to do with the need for a higher front sight only on that type upper.

Carbines have always needed a higher front sight point. It's just that prior to the adoption of the M4 as the standard Army issue weapon, the higher point was incorporated into the front sight blade. My Colt SP1 carbine came w/a higher blade in 1977. As far as I can tell,the F base was created when the M4 was adopted, simply so unit armorers wouldn't have to keep two different blades in stock; one for rifles still in use, and another for the "new" M4.

Good to know, looks like I'll have to get an F-marked  FSB for my bushmaster M4 Barrel so I can use whichever sight I decide on.  ...Gotta do that before I get my Flash hider pinned.

changing the FSB is a real PITA belive me but can be done (try with ADCO).
the cheaper solution is to get a taller front sight post from Bushmaster

+1.  It's not as simple as swapping out sight towers.  The best thing to do would just to get the taller front sight post instead of changing the whole tower.  It would be much cheaper.