Wyhaq shows how to install the PolarStar Airsoft Jack HPA Engine into an AAP-01 GBB pistol so it can use HPA as power source. He also includes entertaining breaks to keep you awake...  "This video I show how to install the popular Polarstar Jack hpa engine into an aap-01. It is actually not that hard to install a polarstar into one of these gbb replicas. It does require some modification in order for it to work, but this guide will easily walk you through step by step to install an hpa polarstar jack engine into an aap01. This guide may also work for the polarstar f1 and polarstar f2, but will not work for the polarstar fusion or the polarstar kythera. This upgrade is great for speedqb and other speedsoft styles of airsoft tournaments and competitions."