Snap cap

A snap cap is a device which is shaped like a standard firearm cartridge but contains no primer, propellant, or projectile.

It is used to ensure that dry firing firearms of certain designs does not cause damage. Some snap caps contain a spring-damped false primer, or one made of plastic, or none at all; the springs or plastic absorb force from the firing pin, allowing the user to safely test the function of the firearm without damaging its components.

A small number of rimfire and centerfire firearms of older design should not be test-fired with the chamber empty, as this can lead to weakening or breakage of the firing pin and increased wear to other components in those firearms. In the instance of a rimfire weapon of primitive design, dry firing can also cause deformation of the chamber edge. For this reason some shooters use a snap cap in an attempt to cushion the weapon's firing pin as it moves forward.

Snap caps and action-proving dummy cartridges also work as a training tool to replace live rounds for loading and unloading drills, as well as training for misfires or other malfunctions, as they function identically to a live "dud" round that has not ignited.

Usually one snap-cap is usable for 300–400 clicks. After that, due to the hole at the false primer, the firing pin does not reach it.

...
Wikipedia