To enable this great cheat so you can progress through the game without having to worry about running out of ammo enter your 'Profile' name as '2ndsquad' (without the quotes).

Unlock Invincibility:
At the 'Chapter Selection' screen hold the Left and Right trigger and press Y, X, B, A, Y, X, B, A.

Unlock Level Select:
At the 'Chapter Selection' Screen hold L + R and press Up(2), Down(2), Left(2), Right(2), X, Right, X, Right, X.

Unlock Supermarine Spitfire:
Defeat Oran on the map.

For unlimited ammo create a Profile called:

2ndsquad