Increases the size of the bore with respect to the diameter of the shot somewhere between 10. Inch caliber metric caliber typical bullet diameter common. Bullets Sizes Calibers And Types Definitive Guide Pew Pew
The .223 Remington was the original ammo used by the AR15/M16. However, due to NATO requirements, it was transformed into the current 5.56x45mm NATO ammo. The change was necessary to meet the NATO requirements: it has to be able to penetrate a World War II U.S. Army M1 helmet at 800 meters. This was also the same requirement for the 7.62mm NATO.
Believe it or not…or actually believe it because it’s true…the 9mm bullet is the same diameter as the bullet used in the .380 and the .38 Special. The difference between the three is the amount of gunpowder behind it and possibly bullet weight. It is the standard round for NATO countries and the majority of police forces around the world.