I've good a new pick for this. ICS MP5. Getting to the gearbox, nice and easy. Two pins, take the stock off, unplug the connector and the lower slides out the back. The rest though? Absolute nightmare. You've got two receiver halves with everything either screwed into them or held between the two halves when they're together, including the wiring. So to reassemble the upper you've got to screw the hop up in, slide the to tube over the inner barrel but not push it on yet, last the wiring with no way to keep it in place, then balance the top part, the rest sight and the mag release in one half and put the second half on and screw it down. Then you've got to push the top tube the test of the way on and put a single screw through the top tube, the receiver halves and the hop up to secure it all and to top it off the tabs secure the receiver sides are long and designed and will absolutely break if you put too much pressure on. Which you need to buy the way because nothing lines up right.