The M4E1 upper receiver is a one-piece design combining the handguard mounting platform with the upper itself. It is machined from 7075-T6 aluminum with M4 feedramps .250 takedown pin holes and supports barrels with a diameter up to 1″. Included is a barrel nut and wrench port door and forward assist. NOTE: This upper is not piston-kit compatible. Requires charging handle barrel handguard gas block muzzle device gas tube and BCG (Bolt Carrier Group) to complete.