Battle of Midway in June, the ships' fighter complement was strengthened to 21 Zeros, and the B5Ns were reduced to 9. By the end of the year, 6 more Zeros replaced an equal number of D3As, giving totals of 27 A6Ms, 12 D3As and 9 B5Ns. Although it was possible to fit all these aircraft into the hangars, 8 or 9 were usually stored on the flight deck to reduce crowding below decks. Armor, armament and sensors As a conversion from an ocean liner, it was not possible to add much armor, although the ships had a double hull. Two plates of Ducol steel, each thick, protected the sides of the ships' machinery spaces. Their aviation gasoline tanks and magazines were protected by one layer of Ducol steel. In addition, their machinery spaces were further subdivided by transverse and longitudinal bulkheads to limit any flooding. The primary armament consisted of a dozen 40-caliber 12.7 cm Type 89 anti-aircraft (AA) guns in twin mounts on sponsons along the sides of the hull. They fired projectiles at a rate between 8 and 14 rounds per minute at a muzzle velocity of ; at 45°, this provided a maximum range of , and a maximum ceiling of . The ships were also initially equipped with eight triple 25 mm Type 96 light AA guns alongside the flight deck. They fired projectiles at a muzzle velocity of ; this provided a maximum range of , and an effective ceiling of at +85°. The maximum effective rate of fire was only between 110 and 120 rounds per minute due to the frequent need to change the fifteen-round magazines. In mid-1943, four more triple mounts were added and another four triple mounts in late 1943–early 1944. Two of these last four mounts were mounted on the stern and the others were placed in front of and behind the island. A dozen single mounts were also added, some of which were portable and could be mounted on tie-down points on the flight deck. After the Battle of the Philippine Sea in June 1944, Jun'yōs anti-aircraft armament was reinforced with three more triple mounts, two twin mounts and 18 single mounts for the 25 mm Type 96 gun. These guns were supplemented by six 28-round AA rocket launchers. Each rocket weighed and had a maximum velocity of . Their maximum range was . In October 1944, Jun'yō had