Depleted uranium is what is left over after uranium is enriched. Enriched uranium can be used in nuclear reactors or nuclear weapons. Depleted uranium is made up mostly of the isotope uranium-238. Uranium-238 is mildly radioactive. It is very dense, or heavy for its size. Because of this, depleted uranium is used in armor piercing bullets and heavy machine guns. Its high density allows weapons to put more energy into a fired bullet which causes more damage to its targets. Bullets made from it will burn when they hit something hard, and their smoke is dangerous to breathe.