A cartridge all encompasses a primer, gun powder, case and bullet. The primer when struck by the firing pin ignites the powder which is enclosed in a brass case creating significant pressure that sends the bullet through the barrel at an extreme rate of speed. Calling a cartridge a bullet only names one part, it would be like calling an automobile an engine or body or gasoline or a human being because it takes the human being from one place to another...poor analogy but I hope you understand. One may also call a cartridge a "round" but never a bullet.
