A Rifle's or Handgun's Caliber
The circles show bore sizes of common calibers. Having the same bore size does not mean different cartridges are interchangeable.
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. However, there is no standard established for designating caliber. In some cases, the caliber is given as the diameter of the bullet, which is the distance between the grooves.
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.
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 size but are designed for different cartridges (the .30-30, .30-06, .308, and the .300 Savage). If you cannot find the caliber stamped on the firearm, take it to a qualified gunsmith.