A nice video from Airsoft Atlanta for those brave enough to convert their GBB airsoft guns to use HPA... "How to DIY custom Polarstar / HPA your airsoft gas gun - we explore the world of tapping your gun to use HPA air tanks. Warning: this is for expert techs only. You may destroy your airsoft gun in the process. We do not recommend you do this to your gun, as it will definitely void your warranty.

This DIY customization requires use of a tap and die set, possibly a drill, and knowledge and experience using a tap and die tool set. You will also need to own the air male nipple, Redline Airsoft air line, and Ninja HPA air tank."