Snap Cap
An inert dummy cartridge sized like a live round, with no primer or powder. Used to safely dry-fire rifles whose firing pins should not strike empty.
Most modern centerfire rifles can dry-fire indefinitely without damaging the firing pin. Some rimfires and older rifle designs need the soft cushion of a snap cap to prevent the pin from peening against the chamber edge.
Snap caps also help with cycling drills. They feed and eject like a real round without firing. Useful for training malfunction-clearing under stress.