Choose head to head(might work in other modes.) When you choose your opponents car quickly hold L1, Square, Circle and Up and Left. Hold until it's finished loading. To use this gun press the horn.
No Mercy Mode(No cops, opponents, or traffic):
At the race selection screen press L1 and R1 together, it should change the head to head to no mercey.
Rally mode(Track covered with dirt):
Select a track and hold L1 and R2
When at the car selection screen hold L1 and R1