Cheat List:
* At the main menu enter X, LEFT, TRIANGLE, SQUARE, X, SELECT for all guns.
* At the title screen enter X, L2, TRIANGLE, R2, SELECT to unlocks all missions.
* Pause the game and enter L1, R2, L2, R1, SELECT for invincibility.
* At the title screen enter R1, L1, L1, R1, R1, R1 to unlock all the maps in quick mission mode.
Source? Here it is.