Aero Precision M4E1 PRO Stripped Lower Receiver, Semi-automatic, 223 Remington/556NATO, Anodized Finish, Olive Drab Green APAR600031AC The M4E1 PRO Lower Receiver seamlessly integrates ambidextrous controls into the M4E1 Lower that allows for the shooter to easily operate the bolt catch, magazine release, and safety without modifying their dominant firing hand. All proprietary componentry of this lower receiver comes preinstalled and is compatible with mil spec parts and components giving the builder the freedom to configure as they see fit.