Ammo is the short, informal word for ammunition: the things you shoot from a gun or similar weapon — bullets, shells, and the like. Soldiers, police, and people who play shooting games say it all the time. In careful writing, ammunition is the full word. Though actually, ammo is wider than “bullets” alone; it is the whole supply of things meant to be fired or exploded. People also stretch it: in an argument, ammo can mean facts you will “fire” at the other side. You would not use ammo in a school essay unless you were quoting how someone spoke.