Also known as: 薬莢
An empty, spent Ammunition casing or Shotgun Shell.
This applies only to the empty casing left over after firing, not to unfired, live Ammunition or intact Cartridges. The shell casing is the hollow metal or plastic tube that originally held the Bullet, gunpowder, and primer before the firearm was discharged.
On Danbooru, these empty 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 dumped from a Revolver cylinder (all at once, typically six at a time).