A snap cap is a device which appears similar to a standard firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 but contains no primer

Percussion cap

The percussion cap, introduced around 1830, was the crucial invention that enabled muzzleloading firearms to fire reliably in any weather.Before this development, firearms used flintlock ignition systems which produced flint-on-steel sparks to ignite a pan of priming powder and thereby fire the...

 or projectile and is used to ensure dry firing

Dry fire

Dry firing is the practice of "firing" a firearm without ammunition. That is, to pull the trigger and allow the hammer or striker to drop on an empty chamber....

older firearms of certain designs does not cause damage. Snap caps contain a spring-damped false primer which absorbs the force from the firing pin allowing the user to safely test the function of the firearm without damaging the components of the firearm.

A small number of rimfire and centerfire weapons of older design should not be test-fired with the chamber empty as this can lead to weakening and possible 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 have usage as a training tool to replace live rounds for loading and unloading drills, as well as training for mis-fires or "jams", as they function identically to a live "dud" round that has not ignited.