. I'm just waiting on a new Gearbox shell from AK2M4 (much credit to Pete, good guy and great site) to put the gearbox back in after my other two shells both have selector plate issues.
The gearbox is going to be a ZCI microswitch shell, built to DMR spec as per "the guide" but to 350fps and retaining full auto like a real Mk12. I'm going to run it standard for a while before deciding on an AB mosfet or a MERF 3.2 and how it will wire into the microswitch trigger.
before you fit all the gearbox components to the ZCI shell put the 2 halves together and check the fit of the hop unit into the front, its almost certain you will need to file a small amount away to get a good fit. I only say this as I didn't first time round and had to dismantle the entire gb in order to correct it and then thoroughly clean out the filings.
The microswitch is a standard electrical component which you can get quite easily from places like Maplins if need be. However note that it is wired so that it fires when the actuator bar is REMOVED from the pressure switch (rather than it having to press the switch as in Ares guns), no biggie, just seems a bit "huh?" until you realise its a clever system that prevents half cycling as juice goes to the motor until the actuator returns to its correct position, presses the switch and cuts power. It works very well. Got 2 of these boxes and not had any problems despite neither having mosfet fitted.