Depleted uranium, also known as Q-metal, D-38 DU, is a toxic and radioactive byproduct created in the production of enriched uranium for nuclear weapons and nuclear energy. It has a radioactive half-life of 4.4 billion years.
The US and NATO militaries began using bullets made of DU in the 1991 Gulf War and have been using it in every major armed conflict since that time. It is used because it is extremely dense (1.6 times as dense as lead) which makes it ideal for piercing heavy armor. The U.S. used 640,000 pounds of DU ammunition just in the first gulf war alone.