A thin copper cup affixed to the base of lead bullets to reduce leading of the barrel by keeping burning propellant gases from melting the base of the bullet when it obturates. Typically used for unjacketed lead pistol bullets in magnum cartridges.
A copper or brass cup used to prevent hot, high-pressure powder gases from deforming the base of lead bullets.
A gas check is a device used in some types of firearms ammunition. Gas checks are used when non-jacketed bullets are used in high pressure cartridges to prevent the buildup of lead in the barrel.