I assume FS means front sight. I use a nut driver to install and remove the FS on Glocks. Are you saying there is just half of the hex head left? If you can't get a grip with a nut driver then just drive it through with a punch. Second option would be a small chisel to cut the head off. Third option, if you don't mind damaging the sight, is to grab it with Vise Grips and yank it off.