Jeidanten got this WWC Excalibur Extreme Performance M4 BUFFER that can be installed on gas blowback M4s from GHK Airsoft, VFC, KWA, Western Arms, WE Airsoft, and Tokyo Marui Gas Blowback. This product is shipped from Taiwan that you can order from the Ruten though it looks like this is available mainly for Taiwan residents. You can enquire if they can ship overseas.