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 different 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.
The term "snap cap" is a generic trademark of the fake cartridge originally introduced by American firearm accessory brand "A-Zoom", and is a word play on percussion cap. It is now used to refer to any accessory product made to perform similar function.

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