Aero's M5E1 Upper Receiver is a one-piece design combining the handguard mounting platform with the upper itself. Aero has condensed the parts needed to mount a free-floated handguard to 8 screws. It features M4 Feedramps, .2795 takedown pin holes and laser engraved T-marks. Comes assembled with Port Door and Forward Assist installed. The M5E1 Upper Receiver is specifically designed to be used with Aero's M5 Enhanced Handguards. It will not work with other AR10/308 handguards. NOTE: Requires charging handle, barrel, handguard, gas block, muzzle device, gas tube and BCG (Bolt Carrier Group) to complete.