At the 'Main' menu press Left, Left, Left, Right, Left, Right, Yellow, Yellow if you want to play the game without having to worry about running out of ammo.

If there is a particular mission you want to play and you don't want to play through the game to get to it then go to the 'Mission Select' screen and hold both the Left and Right Bumpers and quickly enter using the d-pad Left, Left, Right, Right, Y, Y.