you guys nailed it, but i like this subject, so i’ll just add that it’s important to be careful with terminology. the main parts of a cartridge are the projectile (or bullet), the casing (or shell), the propellant (or gunpowder) and the primer (which ignites the propellant). everyone here agrees that the caliber is the diameter of the projectile. in many cartridges the casing is ever-so-slightly bigger than the projectile, except in the case of other sporting or military cartridges, such as the .223 Remington, where the casing is significantly larger than the projectile to house more propellant and thus make the smaller bullet go faster and farther.