page_title: Montana-class battleship
section_title: Anti-aircraft batteries
hierarchical_section_title: Montana-class battleship / Specifications / Armament / Anti-aircraft batteries
caption_attribution_description: Bofors 40&#160;mm anti-aircraft guns on a Mk 12 quadruple mount firing on board USS Hornet (CV-12), circa February 1945, probably during gunnery practice. The original picture caption identifies the photo as having been taken during Task Force 58's raid on Japan, 16 February 1945. However, helmetless members of the gun crew, and rolled up shirt sleeves, strongly indicate that the occasion was in warmer climes and not while in combat. View looks aft on the port side, with the carrier's port quarter 5"/38 guns just beyond the 40&#160;mm mount. Note ready-service ammunition and spent shell casings at right; men passing 4-round clips to loaders at left.[1][2]
context_page_description: The Montana-class battleships were planned as successors of the Iowa class for the United States Navy, to be slower but larger, better armored, and with superior firepower. Five were approved for construction during World War II, but changes in wartime building priorities resulted in their cancellation in favor of continuing production of Essex-class aircraft carriers and Iowa-class battleships before any Montana-class keels were laid.
  Intended armament would have been twelve 16-inch Mark 7 guns in four 3-gun turrets, up from the Iowas' nine Mark 7 guns in three turrets. Unlike the three preceding classes of battleships, the Montana class was designed without any restrictions from treaty limitations. With an increased anti-aircraft capability and substantially thicker armor in all areas, the Montanas would have been the largest, best-protected, and most heavily armed U.S. battleships ever. They were also the only class to rival the Empire of Japan's immense Yamato-class battleships.
  Preliminary design work for the Montana class began before the US entry into World War II. The first two vessels were approved by Congress in 1939 following the passage of the Second Vinson Act.
context_section_description: While the Montana class would not be designed principally for escorting the fast carrier task forces, they would have nonetheless been equipped with a wide array of anti-aircraft guns to protect themselves and other ships (principally the US aircraft carriers) from Japanese fighters and dive bombers. If commissioned, the ships were expected to mount a considerable array of Oerlikon 20 mm and Bofors 40 mm anti-aircraft weapons.
  The Oerlikon 20 mm anti-aircraft cannon was one of the most heavily produced anti-aircraft guns of World War II; the US alone manufactured a total of 124,735 of these guns. When activated in 1941, these guns replaced the .50 in (12.7 mm)/90 cal M2 Browning MG on a one-for-one basis. The Oerlikon 20 mm AA gun remained the primary anti-aircraft weapon of the United States Navy until the introduction of the 40 mm Bofors AA gun in 1943.
  These guns are air-cooled and use a gas blow-back recoil system. Unlike other automatic guns employed during World War II, the barrel of the 20 mm Oerlikon gun does not recoil; the breechblock is never locked against the breech and is actually moving forward when the gun fires. This weapon lacks a counter-recoil brake, as the force of the counter-recoil is checked by recoil from the firing of the next round of ammunition. Between December 1941 and September 1944, 32% of all Japanese aircraft downed were credited to this weapon, with the high point being 48% for the second half of 1942. In 1943, the revolutionary Mark 14 gunsight was introduced, which made these guns even more effective. The 20 mm guns, however, were found to be ineffective against the Japanese kamikaze attacks used during the latter half of World War II. They were subsequently phased out in favor of the heavier 40 mm Bofors AA guns.
  The Bofors 40 mm anti-aircraft gun was used on almost every major warship in the US and UK fleet from about 1943 to 1945. Although a descendant of German, Dutch, and Swedish designs, the Bofors mounts used by the US Navy during World War II had been heavily Americanized to bring the guns up to the standards placed on them by the Navy. This resulted in a gun system set to British standards (now known as the Standard System) with interchangeable ammunition, which simplified the logistics situation for World War II. When coupled with hydraulic couple drives to reduce salt contamination and the Mark 51 director for improved accuracy, the Bofors 40 mm gun became a fearsome adversary, accounting for roughly half of all Japanese aircraft shot down between 1 October 1944 and 1 February 1945.
caption: Bofors 40 mm anti-aircraft guns on a MK 12 quadruple mount fire from the deck of USS Hornet in World War II.
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