Four Fun Facts about the Oerlikon 20mm Antiaircraft Cannon!

The 20mm Oerlikon automatic cannon was a mainstay of United States naval air defense during World War 2, and today we will look at a few of the characteristics and questions that apply to this sort of automatic cannon but not to typical small arms. Like, for instance, how do you cock a gun that has a 400 pound recoil spring? Or, what happens if you fire a high explosive shell into your muzzle cover?


  1. What was the actual performance of the gun? How heavy was the projectile and how fast was it moving? Also, what was the rate of file, rounds per minute?

  2. And to think that this gun was copied by just about every belligerent in the war save for France and the Soviet Union. The French had the HS 404 and the Russians had their own designs whose names I can’t pronounce… and Imperial Japan had a ludicrous number of innovative ideas without the resources to build them. Or, if we must mock the empire, the issue was political backstabbing by factions of the army and the navy. The army’s Type 98 20 mm flak gun was an adaptation of the Type 97 anti-tank rifle for fully automatic operation and it wasn’t too bad when attacking light tanks, infantry, and really slow (and probably lazy or complacent) planes. And if I did my homework right that cannon could also get into a bomber mount and give some rookie P-40 a nightmare/shot through the head (you should never fly right behind a medium or heavy bomber!!). On the other hand the IJN simply copied the Oerlikon and had it mounted in planes while mounting 25mm Hotchkiss guns on ships. And of course the rhetorical question from American analysts was why nobody apart from the IJAAF successfully scaled up the Browning M2 into an auto-cannon (Ho-5). Did I mess up or miss anything?

    • “Russians had their own designs whose names I can’t pronounce…”
      You probably mean 25-мм автоматическая зенитная пушка образца 1940 года if you have problems with that name you can name simpler alternative, that is 72-К.
      Despite attempts to produce similar in caliber (20…25 mm) AA gun was done earlier it all failed. 72-К was adopted before outbreak of Great Patriotic War but due to problems with manufacturer it entered battlefields in significant numbers only in second half of war.
      72-К spawned followed weapons:
      25-мм зенитная пушка обр. 1944 г. (94-КМ) – towed twin mount
      84-К – single naval mount, passed trials, but not produced due to industry evactuaion
      84-КМ – single naval mount, went into production

      25-mm guns were used also in self-propelled form and as armament of armored trains

  3. A few other fun facts.

    5. The reason the Oerlikon was so widely used can be summed up in two words; Antoine Gazda. He was Oerlikon-Buehrle’s demon salesman who sold the gun to practically every major navy and air force in Europe plus the entire British Commonwealth and the United States.

    He did it by swearing blind that he was giving each customer exclusive rights and nobody else had the gun. Naturally, the various Ordnance Departments didn’t talk to each other, it being a classified data situation. It was only after WW2 started in earnest that everybody found out (from capturing each others’ guns) that they’d all been snookered.

    6. The Hispano aircraft cannon used by the RAF, French air force, and etc. was in fact a variant of the Oerlikon. Gazda licensed the French to develop the first aircraft-specific version, and Hispano was the contractor they hired to do the actual work. Clever sod, wasn’t he? In fact, the barrels for RAF Hispano cannon and Royal Navy deck-mounted Oerlikons were pretty much interchangeable; see The Secret War aka Wheezers and Dodgers by Gerald Pawle, in the chapter titled “The Gun from Switzerland”.

    7. According to one of my uncles who served on the USS New Jersey and USS Missouri from 1944 to 1946, another way to cock the gun was to loop half-inch hawser around the cocking handle and have about four burly deck crewmen play tug of war with it. Since the Oerlikon normally had a two-man gun crew (gunner and loader), two crews used to tag-team each other in loading and cocking their guns before it “dropped in the pot”.

    8. The Oerlikon was well thought of in the USN until relatively late in the war, when it was found to be a bit lacking in ability to blow up a Kamikaze before it reached the ship. Even the quad 40mm Bofors sometimes had problems if the suicide plane was fairly sizable, like a Betty or Peggy twin-engined heavy bomber. The latter came in a “Special Attack” version with a one-ton warhead up front where the bombardier usually sat; a Japanese analogue of the German “Mistel” Ju-88 modification, except with a pilot who stayed with it all the way in. Up against monsters like this, the Oerlikon’s reputation came off a bit tarnished, earning it the nickname “door knocker”.

    After the war, of course, the Oerlikon and Bofors both gave way to the semi-automatic twin 3-inch AA mount, as it not only had more range and killing power, but it would also fit in the former quad Bofors gun tub. (To see the twin 3-inch up close, check out the 1964 John Wayne movie In Harm’s Way, directed by Otto Preminger.)



    • As I have pointed out several times before on this forum, the Hispano HS-404 and developments were not API blowback cannons and therefore not variants of the Oerlikon, even if they did share some common design features. The HS-404 inherited those from earlier Hispanos, which were indeed licensed API weapons. The HS-404 was in fact gas-operated, which enabled a lighter bolt and higher rate of fire than contemporary API blowback guns.

      So, at best you could perhaps call the HS-404 an advanced development of the Oerlikon, although I don’t think even that is correct considering the operating system was changed.

  4. It is interesting that the Poles, on a very limited budget, were able to make a far simpler and cheaper version of the Oerlikon – the Polsten. I have read that the Polsten was just as good, made up of fewer than half the parts required for the Oerlikon, and better liked by its crews (mostly Commonwealth).

    • Interesting fact: The Polish Army was small but VERY competent. The only reason Poland got stomped was because the USSR invaded at the same time the Germans invaded. The number of German war dead far outnumbered the entire Polish Army by the time Poland was considered “divvied up between the Nazis and the Commies.” Remember that German troops outnumbered Polish troops 40 to 1, with better tanks and planes.

      • “German troops outnumbered Polish troops 40 to 1”
        Source? Have you concluded this on basis of Ordre de Bataille?

        • Oops I messed up. Polish losses outnumbered German losses. But Poland did not go down quickly nor did it surrender. What Hitler refused to acknowledge was that his forces were sometimes stalled because they had been dealing with Polish units hiding in ambush (let’s just say that the only time Polish cavalry fought German tanks without anti-tank guns ended with the tanks getting grenades thrown into their entry hatches). To say nothing of a regiment that got stabbed to annihilation while sleeping… or am I wrong?

          • Cherndog, you have admirable curiosity in historical implications of use of arms and this is good trait. However, if I may, I’d suggest some prior reading so you do not need to use your usual expression of uncertainty “did I mess up?”. Just friendly hint.

            When comes to Polish campaign and its conduct, without going into prior political connections I can tell of a snippet which is from my own late father’s eyewitness memory. He told me this: “there were fields covered by dead horses”. This was just couple weeks after campaign was concluded.

            What does it mean? That Poles were attacking German armour on horseback? Not at all! What it means is that Polish military of prewar period was heavily relying on cavalry and that is a fact. Of course they were not alone in that time. What is however known is that the tactics and strategy, not to mention use of equipment of German side was superior. For count of casualties you can look at statistics – they are objective enough to be trusted.

          • I wouldn’t say that they were relying heavily on cavalry as such, but the Polish experience from WW1 and the Russo-Polish war was that cavalry was still highly useful on the relatively open terrain that was predominant in Poland. The Polish Army had a fair number of tankettes, armored cars and some light tanks, so it was not nearly as unprepared for modern warfare as German propaganda later suggested. On the flip side the majority of German Army was still traditional leg infantry, which moved slower than the Polish cavalry and could still be surprised even by a traditional cavalry charge while on the march.

            The German superiority came largely from their larger number of tanks, armored cars and good radio communications within the armored units, whereas the Poles still relied on dispatches for mobile communications. German infantry tactics were also generally better, and of course they were able to attain air superiority relatively quickly, although not without considerable losses to German bomber force despite the technologically outdated fighters of the Polish Air Force.

          • “Poles still relied on dispatches for mobile communications”
            This might be also caused by experience of Polish-Soviet war, as during Battle of Warsaw:
            intercepted and deciphered Soviet radio communication provided Polish force advantage

            “technologically outdated fighters of the Polish Air Force”
            Notice that on the other hand it has modern 40mm Bofors AA guns.

          • Radios were quite uncommon in the 1920s in general. The main reason for lack of radios in the Polish Army was simply the fact that Poland did not have much of domestic electronics industry and high quality tactical military radios were generally not for sale in the 1930s. If you couldn’t make them domestically, you probably could not acquire anything close to the needed numbers for tactical use. Even the French Army lacked radios for the same reason and the ones they had were often unreliable.

            Soviet radios were similarly unreliable even before the war, and after the German invasion the production numbers collapsed, since relocating high-precision manufacturing was much more difficult than weapons. For the rest of the war the Soviets relied mainly on American radios and even on stuff like British and American watertight telephone cable for field telephones, since the Soviet domestic cable proved to be very poorly insulated.

      • Is that so?Hitler praised the Poles for their bravery,but he said that they were not organized very well. Rydz-Smigly was a vainglorious fool. In any case,the German cause was just. Those Germans in the Corridor should have been given the right to vote on the issue.And of course Danzig wanted to return to Germany. Fanatic Polish nationalism was the cause of the war,not Hitler’s “demands” !

          • Read AJP Taylor on this and also David Hoggan. The big H wanted an alliance with Poland against Soviet Union.He didn’t hate Poles and never made any unjust demands.Even Henderson said that H’s demands were the most reasonable of those presented by the Germans.

          • Read AJP Taylor and David Hoggan on this issue. Henderson said that Hitler was the most moderate of all Germans on Danzig. He certainly did not hate the Poles,rather he wanted an alliance with them against the Soviets. It was a small war-mongering group in England that pushed Europe into war.

  5. Great subject; both entertaining and learning. It got me going into more reading to refresh something I knew in past, but largely forgotten. This concept of cartridge ignition is really smart and I am not sure if adopted on small arms – yet.

    Also welcome diversion from pistols and rifles. Super job, Ian.

      • Yes Daweo, they mention part of other derived (mostly German) cannons also Russian AGS-30. My question to you would be: is the firing pin separately controlled inside of breech? Is there any diagram to show it? Thanks.

        • “any diagram”
          You might be interested in patent linked to АГС-30 namely 2174661 by Грязев В.П. and Николаев Г.П.:

          it conclusion is that cartridge should be ignited when it is 0.1…0.2 calibers from being fully rammed – less wouldn’t allow significant lower mass of moving parts, more wouldn’t give reliable functioning in all conditions.

        • Hi Denny,
          One of Oerlikon’s patents shows the lever mechanism that positively controls firing pin movement both forward and back

          That is to prevent any slam firing if the firing pin was to break loose.

          Ian’s last Q&A session has a schematic, but my internet connection is not good enough for me to get to that point in the vid.

          In small arms, the Spanish Star z62 SMG had a seesaw mechanism that pushed the firing pin forward as a pin in the bolt face hit the flange on the rear of the barrel.

          There’s no part of the bolt going into the chamber though. So in practice it’s timing of ignition is little different to a fixed firing tit.

          Retraction of the star’s firing pin was by a spring, and there was also a clever inertia lock that kept the bolt from going back if the gun got dropped when the bolt was forward.

          Rebated case heads were patented by Westley Richards / Leslie Taylor, between about 1900 – 1910. Originally for the .425, which had a .540″ diameter case body and a .473″ rim.

          Case body was the same diameter as the .404″ Jeffery, and the .333 and .280 Rigby cases from the nineteen Teens and 7×66 vom Hoffe super Express from the 1950s.

          (All of which get berated and in the case of the German cases, the published velocities get dismissed as probably fraudulent, in Barnes’ ridiculously nationalistic cartridges of America and jonny foreigner’s silly copies… While the super and short mags of the same diameter from Winchester, Remington and Dakota in the 1990s are praised to the heavens with crap like “proven benchrest concept” and “innovative”, and the same velocities accepted as gospel truth, as were the fraudulent velocity claims made about the .458 Winchester).

          There are contemporary Leslie Taylor patents for magazine mods to raise the radically rebated head high enough for reliable feeding in a bolt action, and for extractors and ejectors to handle rebated case heads in double rifles.

          Westley Richards are currently turning out double rifles with those ejectors, and probably turning out double rifles at a greater rate than any of the other British makers.

          • I very much appreciate your note, Keith.
            It is good to keep closer to subject and not to get distracted. There is so-o-o much of stuff out there. My problem is information retention; often I have to go back into items I once knew and their knowledge in time faded. Please stick around!

          • “Case body was the same diameter as the .404″ Jeffery, and the .333 and .280 Rigby cases from the nineteen Teens and 7×66 vom Hoffe super Express from the 1950s.”
            7×66 vom Hofe (single f) is child of .404 Jeffery, it was developed specifically to fit into Husqvarna-Mauser rifle.
            Ernst August vom Hofe (????-1945) developed some high-velocity rifle cartridge in 1930s, along them was 7×73 SE vom Hofe (SE – Super Express) – a necked down .300 H&H Mag.
            In 1940 vom Hofe developed 7×67 SE vom Hofe which would be of shorter overall length, but as Nazi Germany was into war, there was no place for sporting rifle cartridge.
            In 1955 vom Hofe trademark go into possession of Walter Gehmann, high-class sport shooter, weapon and ammunition-designer and introduce 7×66 in 1955.

          • “Barnes’ ridiculously nationalistic cartridges of America”
            So I presume that he didn’t notice that “7mm Remington Magnum” is in fact (bit poorer) version of (earlier) 7×66 SE vom Hofe?
            Notice that vom Hofe has longer neck and difference between overall length and case length is bigger in vom Hofe, additionally 7mm Rem Mag has belted case which is superfluous as shoulders are sharp enough to head-space properly, the only advantage of 7mm Rem Mag is smaller overall length
            According to×66-v-Hofe-S
            7×66 is as follows: 9,0g (139grs) @ 980m/s

          • “Rebated case heads were patented by Westley Richards / Leslie Taylor, between about 1900 – 1910. Originally for the .425, which had a .540″ diameter case body and a .473″ rim.”
            I think, I should remind original purpose of this feature. In most cases it is easier to rebuild repeating rifle for different cartridge if it has identical rim diameter as this originally used, but repeating rifle also have limited overall length of cartridge. If you want significantly increase powder charge you have to increase case volume, but due to limited length it is most commonly done by “fattening” case and rim diameter remain same (for ease of reworking existing fire-arms) its give rebated rim.

          • One of the other advantages of the .425’s rebated rim was charger loading, from an ordinary Mauser stripper.

  6. do you have any of the books or papers for the 20mm i remember reading about the 40mm guns but you don’t have any links to info on the 20 mm documentation.

  7. My father was the gunner on one of these during WWII and he used a Mark 14 “electric” “heated” gunsight. Look it up on Google as it was one of the first analog computers directly afixed to a cannon; though it took three crewmen to run it. My father would tell me all the war stories that he experienced, however it was only two years ago that he confessed to shooting down a Japanese Zero during the battle of Layte Bay in the Phiipeans. He passed away shortly after this confession, so I am unable to gather any further details. I have always wanted to have this cannon, but the capital for the gun and the price for the ammo are beyond my pocket book. Thanks Ian for this video.

  8. I’m not sure people can appreciate how powerful these guns were. A 30-06 rifle cartridge, which is what the M1 Garand fired, threw a 150-grain bullet at about 2800 feet per second. The Oerlikon has similar performance. Its projectile also had a muzzle velocity of around 2800 feet per second. Only the projectile was 3718 grains! Three thousand Seven Hundred!!

    • “Three thousand Seven Hundred!!”
      Because our world has three geometry dimensions, upscaling bullet 2 times, would give 8 times heavier bullet, and 3 times would give 27 times heavier bullet etc.
      Hence assuming 7.62 mm for .30-06 upscaling it to 20 should give ~18 increase of bullet mass that is 2700 grains, but as you noted in reality 3718 grains, that mean 20 mm use different shape of bullet

  9. Oerlikon uses not only API for insured blowback, but also fully supported in chamber impulse backward travel by means of debated rim and parallel sided case. Case sit in the chamber is cut deeper to get full lenght of cartridge with some part of the front section of breechbolt which recoils backward in the chamber when the highest pressure int he barrel exists.

  10. The heavy recoil spring is to accelerate and decelerate the bolt

    It is the inertia of the bolt assembly, and that inertia alone, that resists case backthrust.

  11. Advanced Primer Ignition, impressive action. Was PIAT also API? It has very
    heavy firing spring and ignite mortar cartridge I heard.

    • The PIAT was a spigot mortar and like all mortars, it didn’t have a bolt as such. One function of the heavy spring was to buffer the quite strong recoil. As a mortar, the PIAT was definitely not recoilles weapon, unlike the Bazooka and Panzerschreck, which were rocket launchers and the Panzerfaust, which was a recoilles gun.

  12. My dad served aboard the USS San Jacinto (CVL30) in WWII as an electronics technician whose battle station at General Quarters was in the ship’s Combat Information Center. Dad said that when an incoming aircraft disappeared from the radar screen (so close that its echo merged into the carrier’s on the PPI display) and the 20mm’s opened up, it was time to hit the deck and brace yourself. I’ve seen film of the San Jacinto under kamikaze attack and an attacker hit the ocean just forward of the bow.
    A second or two later and he would have gone right through the CIC. Dad remembered that attack – there was debris around the forward 40mm quad mount afterwards when they stood down and he walked onto the forecastle to get some fresh air.

  13. Ian is right that this was primarily a naval weapon, although the British did arm some of their Crusader AA tanks with it (I’m an old tanker, so I can’t resist)

    Other Crusaders were less tank and more SP artillery, mounting three guns in an open mount

    With the lack of Luftwaffe opposition over Normandy, both were rather quickly discarded, although the Free Polish Armored Division liked using theirs as a form of assault gun.

    World of Tanks fans, time to get the Chieftain moving!

    US Marine Defense(AA)battalions employed it in shore mounts

  14. Great article on the Oerlikon I have a large collection of them and parts bases barrels and shoulder mounts my question is can a serial number be used to trace the history of one of them I have one that appears to be off a sunken ship I bought it from the cannon super store it could be from Pearl Harbor as several items where advertised to be from Pearl Harbor it’s has its based connected to it and is full of sand my firmer wife was yin Gazda her grandad was Antoine Gazda so I have a large collection of history on Antoine Gazda’s Oerlikon production in rhode island

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