Sudden Death, Trial, and Max Power Modes:
To get these modes beat the game on hard. Max power mode gives you infinite ammo.

Trial Mode Alternate:
When it says "press start" press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left, then Square + Start at the same time. Trial mode lets you play any level in the game you want to.

All the Vs. Characters:
At the ''press start'' screen press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left. then press Circle and start at the same time. This will give you all of the versus characters.

Max Power Mode:
At "Press Start" screen press L1, R2, L2, R2, L2, Triangle, Circle, Triangle, Circle, the hold L1 and press START. You must of entered this before it goes to demonstration mode. If you hear a gunshot sound then you've done it correctly. This allows you to start a game with every gun and infinite ammunition.

Long Range Shotgun:
If you need a longer range with the shotgun, this may help to some extent. It is best used when ducking in and out of walls. Target the enemy with your machine gun (it has the longest range) then, immediately switch to the shotgun and it will automatically target the enemy and you will be able to to shoot him.

Trial Mode:
At the "Press Start" screen, quickly press Up, Down, Down, Right, Right, Right, Left, Left, Left, Left, then hold Triangle and press Start to unlock trial mode. If you entered the code correctly, you will hear the sound of a gunshot. This mode allows any level in the game to be played. Note: The code must be entered before demonstration mode begins.

Sudden Death Mode:
At the "Press Start" screen, quickly press L2, R2, L2, R2, Circle, Triangle, Circle, Triangle, then hold L1 and press Start to unlock sudden death mode. If you entered the code correctly, you will hear the sound of a gunshot. This mode allows opponents and your character to be killed with one hit. Note: The code must be entered before demonstration mode begins.

GameShark Codes

(M) Must Be On
EC87E85414392BBC 

Infinite MP3 Mags
3CA3F7381456E788 
  
Infinite Shotgun Ammo
3CA3F7391456E788 

Thanks to Revolution readers Gaseous Snake, Frost2K2, Philip, and Golden Midge!