Aero Precision M4E1 Standard Upper Receiver features an angular machined design that helps it stand apart from standard mil-spec upper receivers. This upper receiver features the same enhanced body profile as the M4E1 Enhanced Upper Receiver but is threaded to accept a standard AR15 barrel nut. It is forged from 7075-T6 Aluminum and precision machined to work with all mil-spec AR15 parts. Features:Enhanced forging that gives the upper receiver a "billet look"Picatinny profile that blends seamlessly with Enhanced and ATLAS Series HandguardsM4 Feedramps.250 takedown pin holes