This URGI/MK16 Kit from RA-Tech should make owners of M4 Gas Blowback Rifles happy as there are versions for most types of GBB tech, whether it be from Marui, WE Airsoft, GHK Airsoft, or KWA. Though it seems they forgot the Western Arms system which is the first M4 GBB tech and is used for G&P and Jing Gong M4 Gas blowbacks.

You can order these at the Airsoft Taiwan online store.