Advanced Airsoft Honolulu explains what PolarStar Airsoft products and other Airsoft HPA products from other brands will work with the Wolverine Airsoft Quake Recoil Stock... "Short answer: Yes to the following - F1, Jack with the standard FCU. No to the Fusion Engine. Yes to N7 if using the Quake Spartan FCU and Yes to Protech if using the Quake Spartan FCU."