Second London Naval Treaty
The choice of calibre was limited by the Second London Naval Treaty, an extension of the Washington Naval Treaty which set limits on the size, armament, and number of battleships constructed by the major powers. After disappointing experiences with the combination of high velocity but relatively light shell in the BL 16 inch /45 naval gun of the s, the British reverted to the combination of lower velocities and (relatively) heavier shells in this weapon.