Shawn McCombs from thief dark has modified his Xbox 360 controller for rapid fire mode by making it fully adjustable. The video shows how the system works. It has a pot that mounts on the side which is used to make the adjustments to how the rapid fire works.
“Now I will have to explain this one to you. You must desolder the three trigger pins shown in the picture below and remove the trigger assembly. Then shorten and bend the middle pin to the outside of the controller and solder a wire to it. Run the wire through the hole displayed below. Then solder the trigger assembly back in, ignoring the middle pin. Then solder a wire to the middle pin pad on the circuit board”.