A lightweight carrier in which a projectile of a smaller caliber is centered so as to permit firing the projectile within a larger-caliber weapon. The carrier fills the bore of the weapon from which the projectile is fired; it is normally discarded a short distance from the muzzle. In short - a badass tank round that you don't want to be hit with.
The M829A1 (nicknamed the "Silver Bullet" by Operation Desert Storm tank crews) is widely regarded as the most effective tank-fired (M1 Abrams 120mm main gun) anti-armor weapon in the world, and overwhelmed Iraqi armor during Operation Desert Storm. The M829A1 is a depleted-uranium long-rod kinetic energy penetrator sabot round capable of defeating heavily armored vehicles.