The VFC HK417 is a very nice airsoft gun with excellent externals. One of the interesting aspects of this gun is the proprietary “Version 2.2” gearbox. This gearbox has different geometry than standard V2 gearboxes, which allows for a steeper grip rake and a much thinner lower receiver. This is in contrast to the upper on the HK417, which is very large and room, including a very large diameter buffer tube.
Converting this gun to use the PolarStar Fusion Engine poses some interesting challenges. Viking from the Soldiers of Sin Airsoft Team sent me his HK417 to take shot at installing the Fusion Engine into the gun, and after an initial false start, I managed to adapt the Fusion Engine to the HK417. The V2.2 gearbox is close in size to the standard V2, except for the pistol grip mount and the cutback at the rear of the gearbox. I initially attempted to modify the V2 Fusion Engine to match the geometry of the V2, but in the end, the fire select mechanism made it impossible to use the Fusion Engine V2 lower assembly.
The HK417 gearbox uses a very unique fire selector plate. With traditional V2 gearboxes, the selector plate moves forward for Safe, and rearward for Full auto. On the HK417, this is reversed. Also the lower receiver of the HK417 is so narrow, the selector plate is very thin and the gearbox essentially sits flush with the sides of the receiver.
Instead of modifying the Fusion Engine, I opted to reuse the HK417 gearbox as a foundation for the Fusion Engine upper cylinder. I cut away the top of the gearbox shell and milled out the inside to make room for the solenoid manifold. The back of the Fusion Engine solenoid manifold also had to be cut down to allow it to fit with the back of the lower receiver.
To support the full auto fire select, a micro lever switch was mounted in the area where the AEG semi auto lever was placed. When the selector plate slides forward it activates the micro switch. There is so little clearance in the lower receiver the micro switch had to be epoxied in place since there was no clearance for any screw heads.
The placement of the Fusion Engine on the gearbox platform is critical as it needs to match up with the hopup and lock it into place. If the hopup isn’t flush with the Fusion Engine and the outer barrel, the magazine won’t seat properly. I learned this the hard way and it resulted having the Fusion Engine mounting holes slotted so it could be adjusted forward. I ended up needed an extra screw to lock the Fusion Engine in place to keep it from sliding backward.
In the end, the Fusion Engine works very nicely in the HK417. Some extreme modifications are needed to the gearbox shell and some extra cuts are needed, but the PolarStar Fusion Engine really brings to life the very nice externals of the HK417 along with all the reliability and tuning capabilities of the PolarStar platform.
I’ve made some changes to the way I convert the HK417. These changes allow the more of the stock gearbox to be used so the hopup is always properly positioned. I also now retain the use of the rear bracket so the Engine will lock into place much better using the standard mount screw under the buffer tube. To better manage the wires I’ve added a small bracket to the back to hold the mini circuit board, but it does make installing the Engine into the lower receiver a little more difficult as the trigger needs to be manipulated to make it drop in. Alternatively you could detach the board from the back of then engine, drop it in, then reattach it.
Since the face of the gearbox is still being used, it requires a custom nozzle as it needs to be ~3mm longer, but a front mounting screw is no longer needed.
I’m actually sticking with the original way I convert HK417s. In the end I believe it’s better to be able to use standard M4 nozzles. I’ve made a few improvements in how the conversion is done, including adding a bracket that holds the mini-circuit board and re-using the rear gearbox bracket so the back screw can still be used.