Fusion Engine™ Drop-In Kit V2 Gen 3 M4/M16The Original HPA Rebel The Fusion Engine was released in 2011 and was the first new HPA system to hit the airsoft market in over two decades. This is the system that started the HPA revolution and has set the benchmark to which all other systems are measured. Although many have followed it, the Fusion Engine is considered by most to be the best. This system won "Best HPA Engine" the very first year Popular Airsoft added the HPA class to their Players'