ahaha I did this when I was 14. Get lots of shims. Lots. Of all sizes. A lot of the quietness and smoothness comes from shimming. A good piston seal is great too but not mandatory. My m4 is genuinely quieter than some of the sniper rifles on the field ahaha.
What make is the gearbox ? , most v3 box's are bomb proof . look at your switch and trolley while your in the box as they can wear out or burn out with none fet'ed guns.
