I believe that what you are referring to is a depleted uranium saboted round. The subcaliber projectile is .224 caliber ( I think). The US military tested Olin's Saboted Light Armor Piercing (SLAP) round in the late 80's but decided not to adopt it in 7.62X51 because it caused catastrophic failures in hot MG barbells. They did make limited purchases in .50BMG, but the stuff is expensive, so it does not show up often.
It is not available to the US public; DU ammo is specifically listed in the federal AP ammo ban. You could probably load saboted rounds at home and get similar velocities, and there are some commercial .308 saboted rounds (non-AP) available