Bullets are to cartridges as flour is to bread as cement is to concrete as a fender is to an automobile, etc.
A cartridge is an assembly of multiple components known as the shellcase, primer, propellant powder and the bullet. Modern firearms are chambered for cartridges, not bullets. Bullets are those little thingies launched by a firearm that fly through the air until they impact a target or eventually fall to the ground. Multiple cartridges are known as ammunition.
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