Frog Engineering Airsoft do some custom HPA conversion of airsoft guns and in this video, they did a conversion of the WE Airsoft 888 Gas Blowback Rifle, WE's version of the HK416D, to use HPA as power source... "Long awaited, HPA conversions for WE HK416/AR15 GBBRs are released! Be sure to check out the latest materials! If you want to get one send message for details!"