If you are building an military M4 clone, the necessities are a KAC M4 RAS (including the KAC VFG), a Matech 600M BUIS, weaponlight (Surefire), and optic (ACOG TA01NSN, ACOG TA31-RCO-M4, Aimpoint Comp M2 w/QRP or LaRue Tactical mount, or an Aimpoint Comp M4). The best source is the Equipment Exchange. I have saved quite a bit going there.