I dont, it works for me... that's what sucks about china brand guns lol.... never good quality control. My Echo1 P90 has CA metal hop up with Prometheus soft bucking and its not rubbing.
I have to put the front receiver first before I put the gear box in. When you put it in, you have to fiddle around with it till you get the best angle for your nozzle, so it wont hit a snag and break your nozzle. Keep trying dude, I know it works. When you find that angle then screw the gearbox in. Best in luck!