Snap Cap
An inert dummy cartridge used for safe dry-fire practice and function checks.
Also: dummy round
Snap caps are inert dummy cartridges with a spring-cushioned primer surface (or a brightly-colored polymer body). They protect firing pins during repeated dry-fire on rimfire and some centerfire firearms, and they’re invaluable for malfunction drills — load a snap cap into a magazine of live rounds (with a partner) to simulate a click-no-bang under pressure.