Sabot
A lightweight carrier that lets a sub-caliber projectile be fired from a larger bore, then separates and falls away at the muzzle.
A sabot is a sleeve or carrier, usually plastic, that wraps a sub-caliber projectile so it can be loaded and fired through a larger bore. The sabot fills the gap between the smaller payload and the barrel, taking the rifling and sealing the gases while the lighter projectile rides along inside it.
Once the assembly clears the muzzle, aerodynamic drag and centrifugal force strip the sabot away and it falls aside, leaving the slender projectile to fly on alone. The appeal is higher velocity from a lighter, smaller-diameter bullet, though the discarding pieces and the precision of separation can add their own challenges to consistency.