| Widely used by the
Allied nations, the Swiss-designed 20mm Oerlikon AA MG 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. These guns were air-cooled and used a gas blow-back
recoil system.
This weapon proved very popular with its ease of maintenance and good rate of fire. In the USN, this weapon replaced the ineffective 0.50" (1.27 cm) MG on a one-for-one basis and was the primary anti-aircraft gun until the Bofors 4 cm became available in large numbers during 1943. In 1944-45, the US found that the 20mm shells were too light to stop the Japanese Kamikaze planes and the higher aircraft speeds made manually controlled guns obsolete. As a result, these weapons were replaced by 40mm Bofors where ever possible and removed from most US ships shortly after the war. Mark I was the original Swiss built version, the USA Mark 2 and British Mark II were the first production version from the respective countries. The differences were mainly in the arrangement of the buffer springs. The later US Mark 4 had a single, heavy buffer spring. Some historical irony: Oerlikon almost went bankrupt in 1935 when the USN rejected their 20mm 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. Unless otherwise noted, the data that follows is for the USA versions, but weapons built by the British had similar performance. |
USA Twin 20mm AA Note here: The USS Texas 20mm guns did not have this gun sight system. It was a manual sighting see photo at top of this page. Secondly it only had one barrel not two. |
| Designation | USA: 20mm/70
(0.79") Marks 2, 3 & 4
British: 20mm/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/minute per barrel |
| 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: 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 included HE and HE-I both with and without tracer. A SAP round was used 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 a twin Bofors. |
|
| Elevation | 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) |
| Designation | USA: Single Mounts Marks 2, 4, 5, 6 and 10 Twin Mounts Triple Mount (not
in service) Quad Mounts Britain Pedestal Mounts Submarine Mounts Twin Mounts (hand
worked) Twin Mounts (power
worked) |
| 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. Mark 5 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 trunion 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 trunion 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. |
|
"Click Here" for additional Information on
Later model 20MM and 40MM guns.