shell casing
The metal or plastic case that holds the bullet, gunpowder, and primer as part of firearm ammunition. Together the parts are called a round or cartridge, though commonly 'round' and 'bullet' are used interchangeably (the bullet is technically only part of a round).
On Danbooru, most often seen empty, being ejected one at a time from a semiautomatic firearm or shotgun, in a rapid sequence from an automatic firearm, or manually ejected from a revolver cylinder (all at once, typically six at a time).
