PolarStar Airsoft JACK Electro-Pneumatic Gearbox Conversion Kit - V2 for M4The JACK system was designed to answer the demand for an affordable, high performance HPA (High Pressure Air) conversion for AEG's. The system is a drop in cylinder replacement that is universally compatible with a wide range of gearbox versions that have center line nozzles. Similar to its sister product, the F1, it replaces the original gearbox components with a programmable pneumatic system allowing for higher performance and greater reliability