Caliber is simply: Barrel (most of them) have rifling with hills (called lands) and valley (called grooves). There isn't any standard for caliber, like I said it's a name for marketing, so the number could be the diameter of the bullet, the diameter at the grooves or the diameter at the lands, or even a round up or down number. For example the .308 Norma Magnum and .300 Norma Magnum actually have the exact same caliber, but one have a round down number as the name to differentiate one from the other. In reality the .300 Norma Magnum is a bigger casing ''neck down'' to fit a smaller .308 caliber bullet. The end result is a light faster bullet, popular for long range shooting.