What is Caliber?

Caliber which is used to describe weapons other than shotguns, has to do with the nominal diameter of the barrel (hence of the ammunition) expressed in hundredths of an inch. Thus a .38 revolver has a barrel .38-inch in diameter. A .30-caliber rifle fires bullets .30-inch in diameter. Magnum used in a description of a firearm refers not to caliber but to the firepower of the propellant used; however, normally a magnum firearm is a slightly different caliber from the closest caliber of a regular firearm--thus, you have .357 magnums which can fire .38 ammunition, but don't try to fire .357 magnum ammunition in a .38 revolver; it will fit the chamber and cylinder, but the firing mechanism and the barrel aren't meant to withstand a magnum load.

Anne Wingate, Ph.D., Science of the Crime, 1992