A shell casing is the outer container of a cartridge used in firearms. It holds the bullet, gunpowder, and primer, which are essential for the ammunition to function. When a gun is fired, the firing pin strikes the primer, igniting the gunpowder and propelling the bullet out of the barrel. After the bullet is fired, the shell casing is ejected from the firearm. It can be collected for various purposes, such as reloading ammunition or forensic analysis in criminal investigations. Shell casings are often made of brass, steel, or aluminum, depending on the type of ammunition and firearm used.