Armour Piercing Discarding Sabot Fin Stabilized (APFSDS) Tank Round
Sub-caliber ammunition is ammunition in which the projectile will have a smaller diameter than that of the bore of the gun from which it is being fired. Firing a sub-caliber projectiles from a gun will typically involve fully or partially encasing the sub-caliber round in a sabot that will have the same diameter as the gun's bore. The sabot centers the sub-caliber in the bore while providing a tight seal that allows the sabot / sub-caliber projectile package to be effectively accelerated by the propellant upon firing.
Typically the sabot will be discarded immediately after exiting the gun barrel, allowing the smaller, and typically more aerodynamic, sub-caliper projectiles to continue on to its target. The combined weight of the sabot and the sub-caliber projectile will typically be much less than that of a full bore round. Assuming that the same amount of propellant is used, the sub-caliber round will be accelerated to to significantly higher muzzle velocity that will give it more range, and in the case of tank rounds more penetrating power.
The goal is to make the sabot as light as possible so that the vast majority of the propellant's energy goes into accelerating the actual damage-dealing projectile, and not the sabot.