Twin & Single 40mm Bofers AA Gun
The USN acquired some of these, mounting them in submarines, destroyers, some lighter vessels and LST's. These USN single mounts were air cooled. Later in 1940, a twin-barrelled example arrived from Sweden via Finland and, using British and Dutch plans, the York Lock & Safe Co. undertook to manufacture them for the Navy, though it did not receive formal licence from Sweden to do so until June of 1941. The first twin Bofors was produced in January of 1942. The naval twin and quadruple mounts were water cooled. Total USA production was about 39,200 weapons. The 40mm proved adequate for all close-in air defense duties until the kamikaze attacks of the last nine months of the war. Each of the twin 40mm's fired 160 rounds-per-minute, per barrel with an approximate effective range of 4000 yards.
The guns were manned by seven men with ammunition passers added to the crew as required. The crew consisted of a gun captain, pointer, trainer, two first loaders and two second loaders. Ready service ammunition was held in racks welded to the "gun tub", a steel bulwark built around the gun to protect the crew. When ammunition in the ready service racks was used up, they would be refilled from below. Despite all this, it was not devastating enough to stop many diving suicide planes from crashing into their targets. Part of the problem was that the 40mm couldn't handle the VT proximity fuse, the 76mm guns being the smallest weapons able to handle that devastating item. Even still, the Bofors 40mm remained the primary close-in anti-aircraft weapon of smaller vessels until the Japanese surrender.
The development of the Mark 51 director system gave the USA weapons greatly improved accuracy.
AMMUNITION--PROVISION FOR READY AMMUNITION AT EACH GUN
20 mm Oerlikon AA Gun
The Bureau of
Ordenance favored a free-swinging anti-aircraft gun, rather than the slower
powered mounts available at the start of the war, which could be used even
without any source of power. The only current free-swinging weapon was the
.50 caliber machine gun, which was lacking in range and firepower. A replacement
was needed that had firepower but still with a low enough weight to be manually
Widely used by the Allied nations, the Swiss-designed 2 cm Oerlikon AA MG
was an outstanding
candidate. It was probably produced in higher numbers than any other AA weapon of WWII.
The USA alone manufactured a total of 124,735 guns. The first USA built
gun was test-fired on 8 June 1941 and 379 had been delivered by 7 December 1941.
Earlier the gun had been
reviewed for service, but rejected on grounds of low rate of fire. Eventual
adoption of the 20mm may have been due to the British who had lost their contact
with Switzerland in 1940 and wanted a U.S. production line. Eventually, on the
grounds of it being the best available weapon, BuOrd approved the production in
Reception was enthusiastic as a
more than welcome replacement for the .50 caliber, production was slow to
quickly replace all of the .50 Cal, so in the early months most air-defenses
continued to rely on the .50-caliber.
Ammunition was held in sixty round drums that fitted on top of the breech. After long periods of firing (about 240 rounds) the barrels had to be changed by the loaders, who wore asbestos gloves. The hot barrels were switched with spare barrels stored in cooling tanks welded alongside the guns. The ammunition drums were stored in lockers behind the gun mounts. When first placed in service in 1942, these guns were mounted singly and were aimed by open ring sights. After 1942, the lead computing stabilized MK14 relative rate gunsight was mounted on these guns for better accuracy, and added the sightrange setter. In 1945, the twin mount became common, which added two more loaders.
The career of
the 20mm was quite short. It had extraordinary successes in 1942, 1943 and up to
mid-1944, but with the advent of the kamikaze, the 20mm was not adequate
anymore. It was, however, retained because of the psychological effect that the
ability to fire at the attacker had for the seamen, and twin and quadruple
mounts had been tested and installed aboard several ships.
Between December 1941 and September 1944, 32% of all Japanese aircraft downed were credited to this weapon, with the high point being 48.3% for the second half of 1942. In 1943 the revolutionary Mark 14 Gunsight was introduced which made these guns even more effective. This gunsight was developed by Dr. Charles Draper of MIT, who calculated that since the guns fired at relatively short ranges, a crude but simple and effective relative-bearing system could be used to control these weapons. The Mark 14 gunsight used two gyros to measure vertical and lateral rate of change, calculated the lead angle to the target aircraft and then projected an off-set aiming point for the gunner. Use of the Mark 14 did require that an electric power connection be provided to the formerly free-standing mountings. This gunsight was later adopted as part of the Mark 51 director which was used to control the 4 cm Bofors, greatly increasing their effectiveness. See the article on the Mark 51 director at our Technical Board for additional information.
In 1944-45, the US found that the 2 cm shells were too light to stop Japanese Kamikaze planes and the higher approach speeds of these planes made manually controlled guns obsolete. As a result, the 2 cm Oerlikons were replaced by 4 cm Bofors where possible during 1944-45 and removed entirely from most US ships by the mid 1950s.
Some historical irony: Oerlikon almost went bankrupt in 1935 when the USN rejected their 2 cm Model 1934 weapon because of its low rate of fire (265 rpm). Only the Japanese Navy's purchase of this weapon saved the company and permitted it to perform further development work which resulted in the much more successful model used during WWII.
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