In stock right now at eHobby Asia is the Spear Arms Tokyo Marui M4A1 MWS Barrel Nut Adapter so you can install other rail systems on it... "The Spear Arms CNC Steel Barrel Nut Adapter for Marui M4A1 GBB Rifle is constructed of steel material. It gives a secure and solid installation of the barrel nut & outer barrel. This includes a washer set to ensure a perfect fit between barrel nut & upper receiver and a reinforced barrel for the Tokyo Marui M4A1 Series Gas Blow Back MWS Rifle."