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bore. 1  A hole or passage made by or as if by use of a drill. 2  A hollow, usually cylindrical chamber or barrel, as of a firearm. 3  The interior diameter of a hole, tube, or cylinder. 4  The caliber of a firearm. 5  A drilling tool.
fusil ouvert image by YvesBonnet from Fotolia.com. The term bore diameter is most commonly used in reference to firearms. Bore is the inside diameter of any cylinder such as a gun barrel. It has a direct bearing on the type of ammunition used in a gun.1  How to Measure a Gauge of Metal Tubing.unsmiths expect their barrels to have a bore diameter with less than a 0.005 inch of error. Although bore is the diameter of the barrel, caliber is the diameter of the bullet. A bullet's caliber is usually denoted as its diameter in hundredths of an inch (a .22- inch diameter bullet is .22 caliber).