Do anyone know how to take the gearbox off VFC HK416?
 

I Stuck with the magazine catch.
Don't really know how to take it off..??

first remove everthing that is holding the mechbox in the reciver out (rear pin, center pin, pistolgrip/motor), then you should be able to press the arm of the mag catch in a bit and just twist off the pushbutten on the other side.
carefull not to brake off the thin metal that stops the magcatch from falling into the magwell.