We currently offer two different lines of HPA conversion systems. There is the Fusion Engine line which is a complete drop-in replacement for an AEG's gearbox. So, you just remove the whole gearbox and drop the Fusion Engine into the rifle in its place. Then we also have our line of cylinder conversion kits which are cylinder replacement systems for an AEG's gearbox. These reuse the original gearbox shell and simply replace the gearbox's internal components.