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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.
The term bore refers to the inside cylinder of a gun's barrel, the tube through which the projectile (bullet or slug) passes through when the gun is fired. The bore may use a rifled or smoothbore design.