Oerlikon 20 mm/70 (0.79") Mark 1
United States of America
20 mm/70 (0.79") Marks 2, 3 & 4
20 mm/70 (0.79") Mark II

Updated 24 September 2006

Widely used by many nations, the Swiss-designed 20 mm Oerlikon automatic weapon was probably produced in higher numbers than any other AA weapon of World War II.  The USA alone manufactured a total of 124,735 guns, with the first one manufactured in the USA being test fired on 8 June 1941.  By 7 December 1941, a total of 379 had been produced.

Oerlikon guns entered service in the British Royal Navy in 1939.  It is not specifically known how many guns were built by Britain and the Dominion nations, but the Mounting Appropriation Lists of September 1945 shows about 55,000 guns in service in those navies.  This total probably includes USA built weapons provided as a part of Lend-Lease and on those ships that had been refitted in US shipyards.  Some British Auxiliary ships still use these weapons even today (2006).

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) BMG on a one-for-one basis and was the primary anti-aircraft gun until the 40 mm Bofors became available in large numbers during 1943.

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 40 mm Bofors, greatly increasing their effectiveness.  See the Technical Board essay on the Mark 51 director for additional information.  Postwar, the Mark 14 was replaced by the Mark 20 Gun Sight, which was a lighter, simpler design.  The Mark 20 was ready to use in ten seconds after being switched on while the Mark 14 took three minutes.

In 1944-45, the USN found that 20 mm 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, Oerlikons were replaced by 40 mm Bofors where ever possible during 1944-45 and removed entirely from most US ships by the mid-1950s.

Use by other nations during World War II:  1) The Italians purchased small numbers of this weapon directly from Oerlikon.  2) Some 2,002 of these guns were sent to the Soviet Union as part of Lend-Lease.  3) An unknown quantity of these guns (Type FF) were purchased by the German Army who designated them as Flak 28 and Flak 29.  These were passed on to the Kriegsmarine in 1939.

Some historical irony:  Oerlikon almost went bankrupt in 1935 when the USN rejected their 20 mm Model 1934 weapon because of its low rate of fire (265 rpm).  Only the Imperial Japanese Navy's purchase of this weapon saved the company and permitted further development work, which resulted in the much more successful version used during World War II.

The Mark 1 was the original design by Oerlikon.  A small number of this version were built in the USA as prototypes and possibly in Britain, as well.  The USA Mark 2 and the British Mark II were the first production versions manufactured in those countries.  The differences from the Mark 1 were mainly in the arrangements of the buffer springs, although the USA Mark 2 also had cooling ribs and two locking slots.  The USA Mark 3 was similar to the Mark 2 but had fewer cooling ribs and only one locking slot.  The later USA Mark 4 was the most common version built in the USA and had a single, heavier buffer spring.  This version was built to slightly different tolerances as it was designed using English measurement units rather than the metric units used on previous Marks.  The Mark 4 Mod 4 had a fluted chamber which allowed easier ejection of the spent cartridge cases.  The Mark 1 could be fired in single-shot mode, while all of the others could only be fired in automatic mode.  All guns used a monobloc barrel and a horizontal sliding breech block mechanism.

These guns are air-cooled and use a gas blow-back recoil system.  This weapon has some unusual features not found in other automatic guns.  The barrel 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 the explosion of the next round of ammunition.

It should be mentioned here how unsuitable the design of the Swiss Mark 1 was for mass production.  Each weapon needed to be tailor made with a great deal of hand fitting during each stage of assembly.  Likewise, the manufacture of individual parts was a long and labor-intensive process.  To give just one example, the barrel spring casing as designed by Oerlikon started as a 56 lbs. (25 kg) solid alloy steel forging.  This required a great deal of machining to produce the finished part which weighed only 6 lbs. (2.7 kg).  BuOrd experts redesigned this piece to consist of a hollow forged base to which a tubular steel extension was welded, thus reducing the starting weight to only 14 lbs. (6.5 kg) with a correspondingly large savings in man-hours, machine tools and costs.  As a result of such redesigns, production time dropped from 428.4 man-hours per gun in 1941 to only 76.2 man-hours in September 1944.

Unless otherwise noted, the data that follows is for the USA versions, but weapons built in Britain and Switzerland had similar performance.


20 mm/70 (0.79") on USS Iowa BB-61 about 1943
The crewman on the left is adjusting the trunnion height
Note the Mark 14 Gunsight and that the censor has obscured the radar antennas on the director and mast
Detail from U.S. Naval Historical Center Photograph # K-16469

Click here for additional pictures
Gun Characteristics
Designation Switzerland (Oerlikon):  20 mm/70 Mark 1

USA:  20 mm/70 (0.79") Marks 2, 3 and 4

British:  20 mm/70 (0.79") Mark II

Ship Class Used On Almost all Allied ships during World War II
Date Of Design about 1939
Date In Service Britain:  1939
US:  1941
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)
Rifling Length 49.1 in (1.246 m)
Grooves (9) 0.015 in deep x 0.205 (0.38 x 5.207 mm)
Twist Uniform RH 1 in 36
Chamber Volume 2.127 in3 (34.855 cm3)
Rate Of Fire Cyclic:  450 rounds per minute
Practical:  Between 250 to 320 rounds per minute
Note:  Some references describe the British weapons as 65 calibers long, but gun data sheets show them to be the same length as USN weapons, 70 calibers.
Type Fixed
Projectile Types and Weights USA
   HE Mark 3 - 0.271 lbs. (0.123 kg)
   HE-I Mark 3 - 0.271 lbs. (0.123 kg)
   HE-T Mark 4 - 0.262 lbs. (0.117 kg)
   HE-T Mark 7 - 0.271 lbs. (0.123 kg)
   AP-T Mark 9 - 0.269 lbs. (0.122 kg)

   HE - 0.272 lbs. (0.123 kg)
   HE-I - N/A
   SAP - N/A

Weight of Complete Round 8.5 oz (0.241 kg)
Bursting Charge USA
   HE Mark 3 - 0.024 lbs. (0.011 kg)
   HE-I Mark 3 - 0.017 lbs. (0.008 kg)
   HE-T Mark 4 - 0.010 lbs. (0.005 kg)
   HE-T Mark 7 - 0.010 lbs. (0.005 kg)
   AP-T Mark 9 - None


Projectile Length N/A
Complete round up to 7.18 in (18.2 cm) long
Propellant Charge USA:  0.061 lb. (0.0277 kg) NC tube
Britain:  0.063 lbs. (0.029 kg) NC flake or tube
Brass Cartridge:  0.2 lbs. (0.09 kg)
Cartridge 20 mm x 110RB
Muzzle Velocity New Gun:  2,770 fps (844 mps)
Average Gun:  2,725 fps (835 mps)
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
Ammunition stowage per gun N/A

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.  A later version held 100 rounds, but it is not known if this actually entered wartime service.

3) The tracer burned for about 3.75 seconds.

4) Steel-cased ammunition had to be greased to allow reliable ejection of the spent cartridge case.

5) Projectiles leave the barrel rotating at about 1,154 RPS.

6) "RB" in the cartridge designation means Rebated Rim.

7) By V-J day, the USA had produced over a billion rounds (1,000,000,000) of ammunition at a cost of $786,791,000.

Elevation With 0.271 lbs. (0.123 kg) HE-I 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
(see Notes 1 and 2)

Single Mounts
   Marks 2, 4, 5, 6, 10 and 16

Twin Mounts
   Marks 20 and 24
   Mark 25 (non-Oerlikon - see Note 7)

Triple Mount
   Mark 23 (not in service - see Note 6)

Quad Mounts
   Mark 15
   Marks 22 and 26 (non-Oerlikons - see Note 7)


Single Mounts
   Marks 1, IA, II

Pedestal Mounts

Submarine Mounts
   Marks IIAS/M  and VIIA* S/M

Twin Mounts (hand worked)
   Marks IX, XIA, XIIA

Twin Mounts (power worked)
   Mark V, Mark VC

Weight USA
   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 16:  N/A
   Mark 20:  1,340 lbs. (608 kg)
   Mark 24:  1,400 lbs. (635 kg)

   Single guns:  1,146 to 1,680 lbs. (520 to 762 kg)
   Light-weight Mark VIIIA:  616 lbs. (279 kg)

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

   All except Mark VIIA and Mark VIIIA:  -5 / +85 degrees
   Mark VIIA:  -15 / +85 degrees or -15 / +75 degrees
   Mark VIIIA:  -15 / +20 degrees

Elevation Rate Most mountings:  Manual Elevation, only
Mark V and VC:  N/A
(see Note 8)
360 degrees
Train Rate Most mountings:  Manual Training, only
Mark V and VC:  N/A
Gun recoil N/A

1) British Marks I, Ia, II had provision for mechanical height adjustment for ease of the gunner.  British Marks IIA, IIIA, VIIA and RCN Mark V were fixed-height pedestal mountings.  The USN Mark 5 mounting was a British design and all but 916 of the 6,101 produced were shipped to Britain.

2) USN Marks 2 and 4 had provision for mechanical height adjustment for ease of the gunner.  Marks 5 and 9 were fixed-height pedestal mountings.  Mark 6 was an attempt at a more reliable mounting and had hydraulic trunnion height adjustment.  Mark 10 was a lightweight design.  Mark 15 was a powered quad mount originally designed for PT boats, but this was not considered to be a serviceable design.  However, the battleships Massachusetts (BB-59), Maryland (BB-46), Washington (BB-55), Colorado (BB-45) and West Virginia (BB-48) each carried one of these mounts during 1945.  Mark 20 was the prototype for twin mountings.  Mark 24 was the standard twin mount in service near the end of World War II.  Marks 5, 10, 20 and 24 all had fixed trunnion heights.

3) The British Mark XIV quad mounting employed guns modified to use belt ammunition, but this design was abandoned in favor of the twin Bofors.

4) British Mark V and Mark VC Twin mounts were powered by an electro-hydraulic pump located off-mount, except for those used on coastal craft, which were powered by the main or auxiliary engines.  These Mark V twin mountings were later adapted for use as 2-pdr, 40 mm Bofors and 6-pdr single mountings.

5) The gun axis on the twin mountings were 13 inches (33 cm) apart for both British and USN designs.  British Oerlikon guns on the Mark VIIa mounting are still in use today on a few Auxiliary ships.

6) The USN Mark 23 triple mount was designed at Pearl Harbor.  50 units were ordered, but testing aboard USS Enterprise CV-6 during 1943 showed that the center gun was difficult to load and the order was cancelled in May 1944.

7) There were also 20 mm (0.79") T31 aircraft guns - not Oerlikons - used on various USN ships during the war.  Mountings included the Mark 22, which was a power-operated Maxson quad mount with the operator seated within the mount and was used on several ships during 1944-45.  This was replaced in 1946 by the Mark 26, also by Maxson, which had a self-contained power source.  The Mark 25 was a twin mounting by Emerson which used T31 aircraft guns.

8) Manually controlled mountings rarely had mechanical stops to prevent unintentional firing angles.  Instead, safety rails were installed around the guns to keep the barrels from pointing into the ship.

9) In an effort to reduce top weight late in the war, many US ships replaced all of their single mounts with half that number of twin mounts.  This kept the number of barrels the same, but reduced manning as well as equipment weight.

Data from
"Naval Weapons of World War Two" by John Campbell
"US Naval Weapons" by Norman Friedman
"Small Arms, Artillery & Special Weapons of the Third Reich" by Terry Gander
"Jane's Ammunition Handbook:  Ninth Edition 2000-2001" edited by Terry J. Gander and Charles Q. Cutshaw
"Battleships:  United States Battleships, 1935-1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"British Battleships of World War Two" by Alan Raven and John Roberts
"U.S. Navy Bureau of Ordnance in World War II" by Lt. Cmdr. Buford Rowland, USNR, and Lt. William B. Boyd, USNR
"Warship 1990" article by Robert F. Sumrall
"Rapid Fire" by Anthony G. Williams
"Naval Ordnance and Gunnery - 1952" Navpers 16116-B
Special Help from Cliff McMullen, Robert Hurst and Vladimir Yakubov