Unlike AR15s, the 308 AR variants do not have an industry wide specification. Therefore, home builds are proving problematic for even veteran AR15 assemblers. Avoid the hassle and buy this factory-assembled complete upper-receiver. Just add your choice of Aero Precision complete lower (or build one to suit your taste), and an Aero Precision bolt-carrier group to get to shooting. (Does not include BCG or Charging Handle