First - good name for a game!/Gunner

Level select 
At the main menu, hold Select and quickly press 
Triangle(3). Then, press Square, X, Square, Triangle, X, 
Triangle, Circle, Square. 

Begin a new game, then hold Select and press Left or Right 
at the mission introduction screen to choose a new starting 
level.

Invincibility 
At the main menu, hold Select and quickly press 
Triangle(3). Then, press Triangle, Circle(2), Triangle, 
Circle, Square, X(2).

Super gun 
At the main menu, hold Select and quickly press 
Triangle(3). Then, press X, Circle, Triangle, X(2), 
Circle, X, Square.

Invisibility 
At the main menu, hold Select and quickly press 
Triangle(3). Then, press Square, Triangle, X, Square, 
Triangle, Circle, X, Triangle.

Overhead view 
At the main menu, hold Select and quickly press 
Triangle(3). Then, press Triangle(2), X(2), Square, Circle, 
Square, X.