When using some non-standard M4 triggers on the PolarStar V2 Fusion Engine, it may have slightly different dimensions that allow the trigger to potentially damage the trigger micro switch. The M4 trigger has a “tail” that is used to prevent the trigger from pushing too far into the trigger micro switch. The tail is contained within a pocket in the Fusion Engine. Some triggers, though, have this tail placed in a different location or angle, so it does not prevent the trigger from pushing too far into the micro switch. This will eventually break the micro switch and cause it to fail. Notably this happens with the G&P Mk23 Stoner 63 trigger, and the G&P Mk46 and M249 triggers. These guns use an external mechanism to safety lock the trigger, so the safety arm in the Fusion Engine is not used.
The internal safety arm can be removed and a hard stop can be added to prevent the the trigger from crushing the trigger micro switch. By simply adding a bolt with a collar to the trigger area, it can function as a hard stop (kudos to PolarStar, where I first saw this technique used in an Mk46 they fixed up).
- Using a 3mm socket head screw, apply a thin layer of grease to the head of the screw.
- Pull and hold the trigger after it fully engages the trigger micro switch.
- Turn the 3mm screw upside down and press it into the trigger area back wall, with the head flush up against the trigger arm. The grease on the head will make an “O” mark on the back wall.
- Mark the center of the grease “O” mark, and drill and tap a 3mm hole at that location. Make sure to remove the selector plate before drilling.
- Use a collar to elevate the head of the 3mm socket head screw. I used a 3mm nut and a small lock washer.
- Tighten the screw in place, and make sure it does not protrude out the other (left) side of the Fusion Engine. It needs to sit flush or it will interfere with the selector plate (if one is being used).
- Reinstall the trigger and trigger cover and test to see if the micro switch is being activated.
- If the screw is too close to the trigger and the micro switch is not being activated, you can sand/grind down the face of the trigger stop to all ow it to reach the switch. If the screw is too far from the trigger and the micro switch can still be pushed too far, you can add layers of heat shrink wrap to the head of the screw to make up the distance.