Bullet casings are the metal shells that hold the components of a cartridge, including the bullet, gunpowder, and primer. When a firearm is fired, the casing is ejected from the weapon, often left at the scene. Casings are typically made from brass, steel, or aluminum, and their design can vary depending on the type of firearm, such as handguns or rifles. The primary function of a bullet casing is to contain the pressure generated when the gunpowder ignites, allowing the bullet to be propelled forward. Each casing is marked with specific information, such as the caliber and manufacturer, which can help in forensic investigations. Understanding bullet casings is essential in the study of ballistics and crime scene analysis.