Press Square, Circle, R1, L1 at the start menu for all weapons with unlimited ammunition
Press Start to pause game play, then press L1, L2, R1, R2. Alternately, hold Triangle + L1 + L2 + R1 + R2 at the title screen. Sarge will say "Retreat" to confirm correct code entry.
Press and hold Square+L1+R1 at the Main Menu. Keep buttons pressed and enter Up, Down, Left, Right. Until you hear a sound. All Levels are now listed in the campaign mode.
If you want to unlock single levels use the following passwords.
Inside Wall CLSNGN
Tan Base BDBZ
Blue Town SMLLVLL
Pool Table FSTNLS