The KWA LM4 Bolt Assembly is a drop-in bolt carrier group that fits the KWA LM4 PTR line, including the LM4C and LM4 RIS. This assembly comes with the nylon polymer bolt and metal Bolt Carrier Key pre-installed, and also includes the metal piston stop insert. This is the stock BCG that comes in all LM4 and AR-15 models and is rated at around 380-400 FPS*