Aero Precision M4E1, Semi-automatic, Stripped Lower Receiver, 223 Remington/556NATO, Anodized Finish, Black APAR600001C The M4E1 Lower Receiver delivers billet aesthetics in a forged package. This custom designed enhanced forging is machined from 7075-T6 Aluminum and compatible with all mil-spec AR15 parts. Aside from the visual upgrades this custom engineered design provides that challenge much more expensive billet options, they have added several functional features to the lower receiver to simplify the assembly process for the at home builder.