The cartridge is the combination of components that cause the firing of a projectile by means of a firearm. Namely: the case, the primer, the propellant charge, and the bullet(s). In the case of a rifle cartridge, the hunter also refers to the bullet.
Cartridges come in different calibers. It is important that the cartridge must always correspond to the caliber of the firearm if it is to be fired successfully. Occasionally there are exceptions, where calibers "related" to each other can be fired from one and the same firearm.