Here are some pictures of a Polarstar Fusion Engine installed into a VFC MP5. This version of the MP5 has the extra 3 round burst mode, but since the FE doesn’t currently support the extra fire mode, the conversion locks out the extra fire select position.
The main issues that have to be addressed are the selector plate, the trigger and the routing of the wire harness to the front. The selector plate is slotted so the cog on the fire select switch can move the plate back and forth. The fire select cog is rotated 180 degrees so it is now at the top to reach the plate. There are two screws added to the body which prevent the fire select switch from over rotating in either direction.
The trigger needs to be moved from the stock gearbox to the Fusion Engine. Unfortunately the tail on the trigger doesn’t reach the hard stop in the FE lower assembly, so a screw was added to fill the gap. It is important that the trigger have a hard stop to prevent over travel from the trigger lever which can damage the Fusion Engine trigger microswitch.
The last issue is running the wire harness to the fore grip area (if you choose to put the FCU and battery in the front). There is very little room in the gun for the wiring, so the hopup block needs to be removed and milled/dremeled out to make clearance for the wire harness connector to pass through.
The Fusion Engine nozzle length (measured at 42.25mm) is basically the same as the M249 nozzle, but the M249 nozzle doesn’t have a chamfered nose, so a custom nozzle is needed, or the M249 nozzle can be modified to work. It’s may also work with an AK nozzle, which might be a tiny bit too long, so it may need to be cut down a little.