Barrels
How to choose between "F" marked and Non "F" marked front sight base
The general rule if you have a receiver with an attached carry handle- the M16A1, C7, C8, Slick side, or M16A2 Receiver you are going to wanting to get a non "F" marked Front sight base.
If you have an M4 receiver, flat top, or detachable carry handle you are going to want the later version of the front sight base which is the "F" marked.