Quoted:
Correct, there is no need for the "F" on a rifle.  A regular FSB will work, but so will an "F".  Although I don't have one, according to Colt the current M16A4 rifle uses the "F" FSB - it's easier to standardize the same parts for everything.

Here is why the both work.  The tolerances in the flat top carbine with the regular FSB were close, sometimes they worked, sometimes they didn't.  The taller shelf "F" made sure they worked all of the time.  If you move the FSB farther out to rifle length you would have to screw the sight post down into the FSB to raise the POI.  There is plenty of room in either sight base to screw the sight post down to accommodate this.  So their was never an issue with the regular FSB on a flat top rifle.  But, it's easier to keep them all the same and use the "F" on the rifle, too.

Remember these are for military applications.  Civilians have the luxury of purchasing and changing parts if something isn't right.  Mixed up receivers and FSB's can always be corrected with a taller sight post, etc.

I hope this isn't confusing you.   For your original question you should use the "F" and it really doesn't matter what "F" stands for.

Quoted:
From what I understand, if it's a 20 inch barrel you don't need a taller front sight post because the longer the barrel the more you have to lower the front sight post in order raise the point of impact.

From a thread I started a few days ago...



ETA: If you have a carbine however, you will need a taller front site post if you don't already have an F-marked front sight base.

Quoted:

I thought I remembered reading that you have to get a longer front sight post if you have a detachable carry handle, any truth to this?