 The main armament of the "Powerful"-class cruisers consisted of two 40-calibre BL Mk VIII guns in single gun turrets, one each fore and aft of the superstructure.The mounts could elevate up to +15° and the guns fired a projectile with a muzzle velocity of to a maximum range of .Their secondary armament of a dozen 40-calibre quick-firing (QF) Mk I or II guns was arranged in casemates amidships.