Sabot
A sabot is a device used in a firearm or cannon to fire a projectile that is smaller than the bore diameter. Since a strong seal is needed to trap propellant gasses behind the projectile, and keep the projectile centered in the barrel, something is needed to fill the gap between projectile and barrel. This is what a sabot does. Made of some lightweight material (usually plastic in smallbore guns, and aluminum in cannon), the stabot usually consists of several pieces held in place by the cartridge or a loose connection. When the projectile is fired, the sabot blocks the gas, and accelerates the projectile down the barrel. When the sabot reaches the end of the barrel, the shock of hitting still air pulls the parts of the sabot away from the projectile, allowing the projectile to continue in flight.For reasons why a smaller diameter projectile is desireable, see external ballistics and terminal ballistics.