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.

Level Passwords:
If you want to unlock single levels use the following passwords.

Bridge FLLNGDWN
Fridge GTMLK
Freezer CHLLBB
Inside Wall CLSNGN
Graveyard DGTHS
Castle FRNKNSTN
Tan Base BDBZ
Revenge LBBCK
Desk DSKJB
Bed STSLP
Blue Town SMLLVLL
Cashier CHRGT
Train NTBRT
Rockets RDGLR
Pool Table FSTNLS
Pinball WHSWZRD