How do I differentiate between bullet sizes?

Bullets are distinguished by both the width of the bullet nose and the length of the cartridge. Rifle rounds are usually longer than pistol ammunition, but they are frequently narrower.

Early American and English bullets were given titles that corresponded to their widths in hundredths of an inch. Thus, a .45 caliber round is just under ½ inch in diameter. European ammunition is normally expressed in the metric system. For example, one of the most widely used rounds is the German-developed 9 millimeter. Another distinguishing feature of ammunition is the presence or absence of a rim at the base of the cartridge. This determines whether it is rimfire or rimless ammunition.