Metallic uranium occurs naturally in tiny quantities. It is a poisonous heavy metal like lead or mercury but only slightly radioactive. You could hold a piece of DU for days without serious exposure to radiation, but swallowing or inhaling it is more dangerous. When metal meets metal at five times the speed of sound strange things happen. Hard ened steel shatters like glass.
Metal flows like putty, or simply vaporises.
A faster shell does not necessarily go through more armour, but, like a pebble thrown into the water, it makes a bigger splash. Armour penetration is increased by concentrating the force of a shell into as small an area as possible, so the projectiles tend to resemble giant darts. The denser the projectile, the harder the impact for any given size. DU is almost twice as dense as lead, making it highly suitable. The other metal used for anti-tank rounds is tungsten, which is also very hard and dense.