The 9mm Luger is also known as the 9mm Parabellum. The name Parabellum comes from the Latin phrase Si vis pacem, para bellum. It translates into English as “If you want peace, prepare for war.”
So, what is the difference between these three calibers: 9mm Parabellum, 9mm Makarov, and .380 caliber? They are all the same caliber if caliber is measured by the inside diameter of the barrel. But they are chambered differently in that the length of the chamber, and hence the brass case each uses, are different. Measured in millimeters, the 9mm Parabellum (or Luger) is 9×19, the Makarov is 9×18, and the .380 is 9×17. These are not precise. The 9mm Luger case is 19.15, Makarov case is 18.1mm, and the .380 is 17.3mm.
One should not try to own more than one of these calibers because of the ease with which their different ammo requirements are confused and can lead to loading the wrong ammo in your gun. That’s not to say you should only own one gun, just that if you own one of these 9mm guns all your other 9mm guns should be the same. Otherwise, you have to be careful not to mix up the ammo, which is easy to do.
When a caliber is expressed in inches instead of millimeters, the number is not exact. For example, the .38 special is not .38 inches. It is .357 inches. The .357 magnum is more precise. The difference is the length of the case and hence the amount of gun powder, more being in the .357. Another thing to know about these calibers. Ammo in .38 special can be fired in a .357 revolver but .357 magnum cannot be fired in a .38 special revolver.
Then there’s the .38 Super, a wonderful firearm. I won’t go into it here. Just remember that it’s altogether a different sort of cat.
If you’re just learning about guns you can see that pretty soon this stuff goes beyond trivia. It becomes knowledge, and it’s important to your safety that you master it.