The U.S. military uses the DU(depleted uranium) round as a tank buster.
It's a saboted round that leaves the muzzle at 1500 m/sec. After leaving
the muzzle, the sabots fly off like a shot cup in a shotgun round. This
sends a dense, relatively "needle-like" projectile with a high ballistic
coefficient flying into the enemy tank. The kinetic energy damages the
enemy tank severly, often blowing off the turret. The DU ignites after impact
creating uranium oxide. IIRC, the poisoning from the fine metal dust and
and oxide was referred as "Gulf War Syndrome". I don't know of any rifle
bullets constructed of DU.