Pause the game, then press R1, Circle, L1, Square, Circle, R1, Right, Right, Circle, Right. The game will return to the main menu if entered correctly. The "Unlimited Ammo" option will now appear.
Unlock All Artwork:
Pause the game, then press R1, Square, R1, Square, Circle, Left, R1, Left, Left. The game will return to the main menu if entered correctly. Enter the "Extras" menu to see an option to unlock all artwork.
Unlock All Levels:
At the level select screen, hold Up on controller 2 then press Start, Select, Select, Square on controller 1.