USS Guam - Ship Specifications - 20mm Light AA Guns

USS Guam (CB-2)

20mm/70 cal Oerlikon

Probably the highest produced AA weapon in World War II, the 20mm Oerlikon machine gun had a good rate of fire, and easy maintenance. It directly replaced the 0.50" machine gun, which lacked range and firepower to be a sufficent AA gun. Originally designed by Oerlikon, a Swiss company, the gun was eventually produced in the US, where factories produced more than 124,000 guns.
This guns success was incredible, accounting for 32% of all downed Japanese aircraft between December 1941 and September 1944. The Mark 14 gunsight, designed by MIT's Dr. Charles Draper, accounted for a great portion of this success. This gun sight calculated the lead angle and two gyros were used measure the vertical and lateral change. This simple and effective sight eventually became a part of the Mark 51 director found on the 40mm Bofors guns.
Towards the end of WWII, the 20mm Oerlikon was being replaced by the 40mm Bofors, because the 20mm shells were too light to be effective against Japanese Kamikaze planes. Also, with aircraft speeds increasing, manually controlled guns, like the 20mm Oerlikon, were becoming more ineffective. Not long after the war, most 20mm guns were removed from US Navy ships.

Technical Data

Gun Characteristics

Designation USA: 2 cm/70 (0.79") Marks 2, 3 & 4
British: 2 cm/65 (0.79") Mark II

Ship Class Used On Almost all

Date Of Design about 1939

Date In Service 1942

Gun Weight 150 lbs. (68.04 kg) (including breech mechanism)
46 lbs. (20.865 kg) (without breech)

Gun Length oa 87 in (2.210 m)

Bore Length 55.1 in (1.400 m)

Number Of Grooves 9

Length Of Rifling N/A

Chamber Volume N/A

Rate Of Fire 450 rounds per minute per barrel

Ammunition

Type Fixed

Weight of Complete Round 8.5 oz (0.241 kg)

Projectile Types and Weights USA HE - 0.271 lbs. (0.123 kg)
Britain HE - 0.272 lbs. (0.123 kg)

Bursting Charge NA

Projectile Length N/A

Propellant Charge USA: 0.061 lb. (0.0277 kg)
Britain: 0.063 lbs. (0.029 kg) NC
Brass Cartridge: 0.2 lbs. (0.09 kg)

Muzzle Velocity New Gun: 2,770 fps (844 mps)
Average Gun: 2,725 fps (835)

Working Pressure USA: 19.6 tons/in2 (3,090 kg/cm2)
Britain: 20 tons/in2 (3,150 kg/cm2)

Approximate Barrel Life 9,000 rounds

Notes:
1) Outfits in both the USA and Britain included HE and HE-I both with and without tracer. A SAP round was also manufactured in Britain.
2) Spiral magazines held 60 rounds and were spring driven.
3) Britain developed a quad mounting using belt ammunition but this was abandoned in favor of the twin Bofors.

Range

0.271 lbs. (0.123 kg) HE Mark 3 Shells

Range @ 10 degrees 3,450 yards (3,154 m)

Range @ 15 degrees 3,950 yards (3,612 m)

Range @ 20 degrees 4,275 yards (3,909 m)

Range @ 25 degrees 4,525 yards (4,138 m)

Range @ 30 degrees 4,650 yards (4,252 m)

Range @ 35 degrees 4,725 yards (4,320 m)

Range @ 40 degrees 4,775 yards (4,366 m)

Range @ 45 degrees 4,800 yards (4,389 m)

AA Ceiling 10,000 feet (3,048 m)

Mount / Turret Data

Designation USA:
Single Mounts - Marks 2, 4, 5, 6 and 10
Twin Mounts -Marks 20 and 24
Triple Mount (not in service) - Mark 23
Quad Mounts - Marks 15, 22 and 26
Britain
Single Mounts - Marks 1, IA, II
Pedestal Mounts - Mark IIA, IIIA, VRCN, VIIA
Submarine Mounts - Mark IIAS/M and VIIA*S/M
Twin Mounts (hand worked) - IX, XIA, XIIA
Twin Mounts (power worked) - Mark V, Mark VC

Weight Marks 2 and 4: 1,695 lbs. (769 kg)
Mark 5: 1,540 lbs. (699 kg)
Mark 6: 1,691 lbs. (767 kg)
Mark 10: 950 - 1,100 lbs. (431 - 499 kg)
Mark 15: 560 lbs. (254 kg)
Mark 20: 1,340 lbs. (608 kg)
Mark 22: 2,250 lbs. (1,121 kg)
Mark 24: 1,400 lbs. (635 kg)
Mark 26: 3,000 lbs. (1,361 kg)

Elevation Mark 2 and 4: -5 / +87 degrees
Mark 5: -5 / +87 degrees
Mark 6: -15 / +90 degrees
Mark 10: -15 / +90 degrees

Elevation Rate Manual Elevation, only

Train 360 degrees

Train Rate Manual Training, only

Gun recoil N/A

Notes:
1) Mark 2 and 4 had provision for mechanical height adjustment for ease of the gunner. The Mark 5 mounting was a British design and all but 916 of the 6,101 produced were shipped to Britain. Mark 6 was an attempt at a more reliable mounting and had hydraulic trunnion height adjustment. Mark 10 was a lightweight design. Mark 20 was the prototype for twin mountings. Mark 24 was the standard twin mount in service near the end of WWII. Marks 5, 10, 20 and 24 all had fixed trunnion heights.
2) Mark 15 was a quad mount originally designed for PT boats but was not a serviceable design. Mark 22 was a power-operated quad mount with the operator seated within the mount and was used in several ships in 1944-45. Mark 16 was the standard post-WWII mount, in 1982 it was described as unreliable and difficult to maintain and was phased out in favor of the 25mm chain gun.

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Gun Characteristics
Designation	USA: 2 cm/70 (0.79") Marks 2, 3 & 4 British: 2 cm/65 (0.79") Mark II
Ship Class Used On	Almost all
Date Of Design	about 1939
Date In Service	1942
Gun Weight	150 lbs. (68.04 kg) (including breech mechanism) 46 lbs. (20.865 kg) (without breech)
Gun Length oa	87 in (2.210 m)
Bore Length	55.1 in (1.400 m)
Number Of Grooves	9
Length Of Rifling	N/A
Chamber Volume	N/A
Rate Of Fire	450 rounds per minute per barrel

Ammunition
Type	Fixed
Weight of Complete Round	8.5 oz (0.241 kg)
Projectile Types and Weights	USA HE - 0.271 lbs. (0.123 kg) Britain HE - 0.272 lbs. (0.123 kg)
Bursting Charge	NA
Projectile Length	N/A
Propellant Charge	USA: 0.061 lb. (0.0277 kg) Britain: 0.063 lbs. (0.029 kg) NC Brass Cartridge: 0.2 lbs. (0.09 kg)
Muzzle Velocity	New Gun: 2,770 fps (844 mps) Average Gun: 2,725 fps (835)
Working Pressure	USA: 19.6 tons/in2 (3,090 kg/cm2) Britain: 20 tons/in2 (3,150 kg/cm2)
Approximate Barrel Life	9,000 rounds
Notes: 1) Outfits in both the USA and Britain included HE and HE-I both with and without tracer. A SAP round was also manufactured in Britain. 2) Spiral magazines held 60 rounds and were spring driven. 3) Britain developed a quad mounting using belt ammunition but this was abandoned in favor of the twin Bofors.

Range
0.271 lbs. (0.123 kg) HE Mark 3 Shells
Range @ 10 degrees	3,450 yards (3,154 m)
Range @ 15 degrees	3,950 yards (3,612 m)
Range @ 20 degrees	4,275 yards (3,909 m)
Range @ 25 degrees	4,525 yards (4,138 m)
Range @ 30 degrees	4,650 yards (4,252 m)
Range @ 35 degrees	4,725 yards (4,320 m)
Range @ 40 degrees	4,775 yards (4,366 m)
Range @ 45 degrees	4,800 yards (4,389 m)
AA Ceiling	10,000 feet (3,048 m)

Mount / Turret Data
Designation	USA: Single Mounts - Marks 2, 4, 5, 6 and 10 Twin Mounts -Marks 20 and 24 Triple Mount (not in service) - Mark 23 Quad Mounts - Marks 15, 22 and 26 Britain Single Mounts - Marks 1, IA, II Pedestal Mounts - Mark IIA, IIIA, VRCN, VIIA Submarine Mounts - Mark IIAS/M and VIIA*S/M Twin Mounts (hand worked) - IX, XIA, XIIA Twin Mounts (power worked) - Mark V, Mark VC
Weight	Marks 2 and 4: 1,695 lbs. (769 kg) Mark 5: 1,540 lbs. (699 kg) Mark 6: 1,691 lbs. (767 kg) Mark 10: 950 - 1,100 lbs. (431 - 499 kg) Mark 15: 560 lbs. (254 kg) Mark 20: 1,340 lbs. (608 kg) Mark 22: 2,250 lbs. (1,121 kg) Mark 24: 1,400 lbs. (635 kg) Mark 26: 3,000 lbs. (1,361 kg)
Elevation	Mark 2 and 4: -5 / +87 degrees Mark 5: -5 / +87 degrees Mark 6: -15 / +90 degrees Mark 10: -15 / +90 degrees
Elevation Rate	Manual Elevation, only
Train	360 degrees
Train Rate	Manual Training, only
Gun recoil	N/A
Notes: 1) Mark 2 and 4 had provision for mechanical height adjustment for ease of the gunner. The Mark 5 mounting was a British design and all but 916 of the 6,101 produced were shipped to Britain. Mark 6 was an attempt at a more reliable mounting and had hydraulic trunnion height adjustment. Mark 10 was a lightweight design. Mark 20 was the prototype for twin mountings. Mark 24 was the standard twin mount in service near the end of WWII. Marks 5, 10, 20 and 24 all had fixed trunnion heights. 2) Mark 15 was a quad mount originally designed for PT boats but was not a serviceable design. Mark 22 was a power-operated quad mount with the operator seated within the mount and was used in several ships in 1944-45. Mark 16 was the standard post-WWII mount, in 1982 it was described as unreliable and difficult to maintain and was phased out in favor of the 25mm chain gun.