Caliber is the decimalinch diameter of the gun barrel. For example the land to land diameter of a 54 caliber rifle is 54/100's of an inch. The gauge of a gun is the measurement of the bore of the gun in terms of the number of lead balls of the same diameter that would weigh a pound. For example a 20 gauge Northwest Trade gun would require balls of a size such that 20 balls would weigh one pound.
