Unspent shell casing for ammunition.
The shell casing is the brass container that holds the projectile, gunpowder, and primer. When the primer is hit, it ignites the gunpowder inside, which burns. The expanding gasses force the projectile down the barrel and out towards its target. Once its job is fulfilled, the casing is usually ejected from the gun, becoming a popular symbol in modern media to signify assassination.
Can be obtained from spare munitions.