While gameplay, hold L, and then you can enter one of the following codes:
X, O, X, R, Square, Triangle : infinite assualt rifle ammo
R, Triangle, R, O, R, Square: infinite grenade launcher
R, O, Square, R, Square, X: infinite shotgun ammo
O, Triangle, R, R, X, O: infinite SMG ammo
X, R, Triangle, R, Square, R: bulletproof
Square, Circle, X, R, R, Triangle: damage enemy health
Triangle, X, Triangle, Square, R, O: unlock one shot kill
Triangle, X, O, R, Triangle, R: wield excalibur
X, R, O, R, R, Square: wield soul reaver