The term 'bullet casing' refers to the components of a bullet used in firearms. It is composed of two parts: the CARTRIDGE, which contains the propellant and primer, and the SHELL, which houses the bullet itself. The CARTRIDGE is typically made of brass or steel and is 9 letters long, while the SHELL is usually made of brass or copper and is 5 letters long. Together, these two components form the bullet casing, which is essential for the proper functioning of a firearm.