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Caliber is used to describe the size of a rifle or handgun bore and the size of cartridges designed for different bores.

  • Caliber usually is measured as the diameter of the bore from land to opposite land and is expressed in hundredths of an inch, thousandths of an inch, or millimeters. For example, a .270-caliber rifle bore measures 270/1000ths of an inch in diameter between the lands and has a larger bore diameter than a .223-caliber rifle.
  • Caliber designations sometimes have a second number that has nothing to do with the diameter. For example, the popular .30-30 is a .30-caliber cartridge, but the second number is a holdover from the days when the cartridge took 30 grains of powder. The "06" in .30-06 refers to the year (1906) it became the official ammunition of the U.S. military.
  • The caliber designation on an ammunition box refers to the caliber of the firearm the cartridges are to be used in and not to the actual size of the bullet.
  • Every rifle or handgun is designed for a specific cartridge. The ammunition must match the data stamp on the firearm. For example, there are several .30-caliber firearms that use the same bullet diameter but are designed for different cartridges (the .30-30, .30-06, and the .300 Savage). If you cannot find the caliber stamped on the firearm, take it to a qualified gunsmith. The bullet caliber placed on the end of a cartridge is often larger than the firearm caliber, allowing the rifling to "bite" and spin the bullet as it exits the barrel.