Ammunition is packaged in a cartridge, which has four parts: the primer, propellant, case, and projectile (the bullet). A cartridge is often referred to as a ‘round.’ Shotgun cartridges can also be called ‘rounds,’ or ‘shells.’ They are called ‘shotshells’ when they contain pellets instead of a slug or other projectile — but regardless of what you call it, there’s no such thing as a magic bullet.