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. This model has a black hardcoat anodized finish. NOTE: This upper is not piston-kit compatible.