Caliber refers to the diameter of a bullet being fired from a gun. The measurements are taken in hundredths of an inch, meaning a 30-caliber bullet has a casing diameter of .30 inches. The caliber of the bullet does not necessarily provide any information about the load, or the amount and type of powder inside the casing. Variations on length and weight are common within the same caliber casing. Identifying the caliber from the casing is a simple.
Identify the casing of the bullet. This is the lower half of the bullet, usually a brass or copper color.
Turn the bullet so the bottom of the casing is facing you.
Locate the firing pin. This is the small inner circle at the center of the base of the bullet casing.
Read the engraving, known as the "headstamp," found around the outer ridge of the bullet casing surrounding the firing pin.
Locate the number preceded by the decimal point to identify the caliber.
This method works for civilian ammunition. Military ammunition does not designate caliber on the casing, just the plant of manufacture and year.