"Formidable" and her sisters had four 40-calibre Mk IX guns mounted in twin-gun turrets fore and aft; these guns were mounted in circular barbettes that allowed all-around loading or elevation. Shell allowance was 80 rounds per gun. The "Formidable" class were the first to carry these guns, which featured several improvements over the earlier Mk VIII guns used in older battleships, including a stronger barrel design and higher muzzle velocity. "Formidable" and "Implacable" had BVI type mountings for their guns, while "Irresistible" received newer BVII mounts, the primary difference between the two types being the arrangement of the shell hoists and other equipment. Both types of mountings had a range of elevation from -5 degrees to of 13.5 degrees. The BVI mounts required the guns to return to 4.5 degrees to be loaded, while the BVII mounts allowed for loading at any angle. The guns had a muzzle velocity of , and they were capable of penetrating 12 inches of Krupp armour at a range of . At their maximum elevation, the guns had a range of . The guns had a rate of fire of one shot every eighty seconds.