Here is an extremely simple modification to enable you to shoot your Barret .50 cal like a P90.
UPDATE: There seems to a problem with certain types of controllers. There are Matrix controllers that use PWM for the LEDs and some which use a different method. To determine the type of yours, remove the battery pack of your controller. Above the right battery terminal, if there is a " TP-19" then it is matrix controller and it will work with this mod. If there isn't a "TP-19" than it is the newer CG version and it will not work.
Step 1: Ingredients
A TT8 Torx Driver
A Small Push Button Switch
Some Thin Wire, Any will do (I used IDE Cable)
An Xbox 360 Controller (Wired or Wireless, it works the same)
A Drill and Drill bit (To match the diameter of the thread on your switch)
Some form of surface to work on. (I used a window ledge, I suggest you don't)