You will first need to find the gun for which you would like infinite ammunition. This trick works for only one gun type. After selecting a gun, find the exact gun on an ally or enemy and 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 before picking 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 every time.