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A snap cap is a device which is shaped like a standard firearm cartridge but contains no primer, propellant or projectile. It serves the same function as a dummy round, but different in that a dummy is usually made from a real cartridge with its propellant and primer removed (still has the projectile and the casing), while a snap cap is 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® (now a subsidiary under Lyman Products Corp.), and is a word play on percussion cap.
A snap cap is used to ensure that dry-firing firearms of certain designs does not cause stress and impact damage to the firing pin and/or the barrel breech. Some snap caps contain a false primer that is either spring-buffered, made of rubber or soft polymer, or none at all. The springs or plastic will absorb the impact force from the firing pin during dry-firing, allowing the user to practice trigger pull or safely test the function of the firearm action without damaging its components.
Rimfire and a small number of 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, dry firing can also cause deformation of the chamber edge due to the firing pin striking it without the cartridge rim in between. For this reason some shooters use a snap cap in an attempt to cushion the weapon's firing pin as it moves forward. Most commercial snap caps have a polymer/rubber false primer to receive the firing pin strikes, which is usally usable for up to 300 to 400 clicks. After that, due to the impact crater on the false primer becoming deepened from the repeated strikes, the firing pin does not touch and transfer its momentum, and the snap cap loses its cushioning effect. Some higher-end snap caps have a spring-dampened false primer that can last much longer, but also more expensive to make.
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.
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