1 Distance across the bore measured from land to opposite land. In the USA and UK this measurement is usually given in 0.00 or 0.000 of an inch. In Europe and Asia the measurement is generally expressed in millimetres. Commercial designation of a calibre need only be within several thousandths of an inch of the true bore diameter.
2 In ballistics, a unit expressing comparative dimensions; e.g. with a bullet, if the length is three times the diameter it is described as three calibres long.