·  Mission select

At the mission selection screen, hold LB + RB and quickly press Left(2), Right(2), Y(2).

·  Infinite ammunition

You will first need to find the gun for which you would like unlimited ammunition. This trick will only work for one type of gun. After you have chosen a gun, find the exact gun on an ally or enemy, then kill him. After he is dead, his gun will appear on the ground. When walking over the gun it will say you have received that weapon's ammunition. If you have full ammunition, waste a few shots then pick up another of that same gun. However, this time the gun will stay on the ground. You can keep walking over the same gun on the ground repeatedly for more ammunition each time.