Rear sights for AR w/o F marked FSB

[jwsturr]

I have my lower assembled, but I haven't picked out my upper yet. I know that the f marked fsb is taller. But if a get a flat-top upper with a standard fsb, which rear sights work with the standard height fsb? Do I need a fixed or can I use a flip-up?

16" barrel

 

[Runningman]

Knowing your barrel length would have helped. The F marked FSB should be used for the shorter 16" barrel as opposed to the original standard 20" AR barrel that get standard sights.

 

[jwsturr]

16" barrel