"Calibre" is defined as the internal diameter of the barrel. In British and European practice this is measured across the "lands", the part of the barrel which has not been grooved to form the rifling. In American practice it is measured across the "grooves", ie the diameter across the rifling. There is an increasing, and to my mind welcome, tendency to define "calibre" as the external diameter of the bullet. Until that becomes general practice, it is not clear that 6mm, .242, .243, .244 and Weatherby .240 are all the same calibre.
But! Calibre does not necessarily equate with the power o