[Xboxplaya978] has posted instructions detailing how to add two rapid fire trigger buttons to your Xbox controllers. He got the original plans from Mattie on Xbox-scene, but his site has since gone down. The circuit is based on an a-stable multi-vibrator used to flash LEDs. Halo players have been drawn to this mod because it makes using the battle rifle or covenant carbine a lot more enjoyable.
Related: Xbox 360 Hacks, Xbox Hacks