The shell casing is the metal or plastic case that holds the bullet, gunpowder, and primer as part of firearmammunition. Together the parts are called a round or cartridge, though commonly 'round' and 'bullet' are used interchangeably (the bullet is technically only the projectile, not the entire cartridge).
On Danbooru, shell casings are most often seen being ejected one at a time from a semiautomatic firearm or shotgun, in a rapid sequence from a machine gun, or manually ejected from a revolver cylinder (all at once, typically six at a time).
