All Guns Unlocked: 

Hold L+R and press Down, hold L and press C-Left,
hold L and press C-Right, Hold L+R and press C-left,
hold L and press Down, hold L and press C-Down, 
hold R and press C-Left, hold L+R and press C-Right,
hold R and press Up, hold L and press C-Left. 


Maximum Ammo: 

Hold L+R and press C-Right, hold R and press Up, 
hold R and press Down, hold R and press Down, Hold
L+R and press C-Right, hold L+R and press Left, 
Hold R and press Down, hold R and press Up, hold L+R
and press C-Right, Hold R and press Left. 


Invulnerability: 

Hold L and press Down, hold R and press C-Right, 
Hold R and press C-Up, Hold L and press Right, 
hold L and press C-Down, hold R and press C-Up, Hold
L and press Right, hold R and press Down, hold L and
press Left, hold L+R and press C-Right. 


Invisibility: 

Hold R and press C-Left, hold L+R and press C-Up, 
hold L+R and press left, hold L+R and press Up, Hold
R and press Up, hold L and press C-Left, hold R and
press C-Up, hold L and press C-Down, hold L+R and 
press left, hold R and press Right. 


*Note: Enter these codes during a single-player game
(don't pause) with the L and R buttons, C buttons 
and the D-pad (Directional pad). These codes are 
tricky, so make sure to 
hold the buttons for about 2 seconds before going
to the next step of the code.