F.A.B. Defense Flip Up Front Sight, Fits Picatinny Rails, Polymer, Black FX-FBS FBS is an advanced" low-profile" durable" auxiliary front sight. It can be used with any rear sight" but has been designed to provide an extreme optical advantage when paired with the FAB Defense(R) RBS Rear Back-up Sight. Sight pin elevation is easily and quickly adjusted without tools. FBS is readily mounted on any 1913 Picatinny rail" making it ideal for use on M4 / AR-15 / M-16 platforms and on the FAB Defense(R) KPOS.