At the main menu, highlight the "Load Game" option. Hold L3 + R3 and press X to enter the "Select Mission" screen. Choose any starting point from any mission, including debriefing/training. This will not effect your memory card unless you save the game. Press Triangle to exit the screen.
Pro Action Replay Cheats
(M)
0E3C7DF2 1853E59E
EEA95786 BCA99C80
Inf Armour
DE895352 BCA99B83
DE89535E BCA99B83
Lots Of Health
DE8C2BF2 BCA99B83
DE894A5E F8AA1203
DE894A5A 002A9B83
DE894A86 CCA99B86
Disable AI
DE8D588E BF899B8B
DE8D588A BCA99B83
Walk Through Doors
DE8D23EE BCA99B83
DE8D23EA BCA99B83
DE8D23F6 CCA99B8F