There is a more advanced shimming technique that allows you to precisely align the pinion gear(AKA the motor gear) with the bevel gear for optimum performance. This technique requires you to mount the motor to the gearbox while it is outside of the gun in order to observe how the motor engages the bevel gear.
To do this, you would need to attach your gun's pistol grip or motor mount to the gearbox and check the positioning of the bevel gear with the motor engaged. You would then remove the motor and try to replicate that positioning using shims.
I am just mentioning this technique to appease the hardcore tuners out there, but this technique very time consuming and not practical for fast repair shop technicians. However, for those of you who are building high speed or high velocity guns, it is highly recommended.