Snap cap
A snap cap is a firearm accessory device shaped like a standard cartridge/shotshell but contains no functional components, namely the primer, propellant (gunpowder) and projectile (bullet or slug). It serves the same purpose as a dummy round, but differs in that a dummy is usually modified from a real cartridge with its propellant and primer removed (but still has the projectile crimped to the casing), while a snap cap has a monolithic outer shell and is specifically designed to be a fake cartridge from the very beginning.