Aero Precision M4E1, Assembled Upper Receiver, Anodized Finish, Olive Drab Green, Fits AR15 APAR700220AC The M4E1 Threaded Upper Receiver features a new angular machined design that helps it stand apart from standard mil-spec upper receivers. This upper receiver features the same enhanced body profile as their M4E1 Enhanced Upper Receiver but is threaded to accept a standard AR15 barrel nut. It is forged from 7075-T6 Aluminum" precision machined to work with all mil-spec AR15 parts and includes the port door and forward assist pre-installed.