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Oerlikon 20mm manual

The Oerlikon 20mm anti-aircraft cannon was widely used by naval and air forces during World War II, by countries on both sides.  It is a fairly well-recognized weapons, with a characteristic offset 60-round drum magazine. You might think that an automatic 20mm cannon would require a pretty secure locking mechanism to withstand the pressure from firing – but not so. The Oerlikon was a completely unlocked design, using advanced primer ignition and a really stout recoil spring and heavy bolt to create a functional and reliable blowback cannon.

The advanced primer ignition meant that the cartridge was actually fired while still moving into the chamber, so that the recoil had to overcome the forward inertia of the bolt before it could begin to push it backwards. The cartridge used was a straight-walled, rebated rim design, so that it could move smoothly in and out of the chamber. The rebated rim allowed the bolt face to not exceed the diameter of the chamber, allowing it free movement as well.

There are actually some parts kits in the US for these guns, and at least one attempt at rebuilding in semiauto has ended tragically, with a kaboomed gun at Knob Creek years ago. There was a great deal of careful engineering that went into proper balancing of weights, spring power, and timing on the Oerlikon, and redesigning one for semiauto use (or rebuilding one to original spec, for that matter) is not a project for novices. Or experienced builders. I don’t say that to discourage folks from working on high-velocity cannons, just to advise that you look elsewhere for a practical first project.

In the interest of a more complete education, I invite you to take a look at the US Ordnance Pamphlet we have on the Oerlikon (at 255 pages, it’s really more of a technical manual than “pamphlet”):

(1943) Oerlikon 20mm AA gun US Ordnance Pamphlet (English)

For what it’s worth, I’m planning to add a bit more of the mounted and artillery type weapons to the information here at Forgotten Weapons – but I’d like your input. Would you like to see the occasional cannon and field gun, or would you prefer to just have small arms?

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12 comments to Oerlikon 20mm manual

  • Al

    Congratulations on this website. I vote for the option “yes, big or small, all are interesting”because I love the weapons engineering and engineering in general. Small arms are more complex than large ones, but in the big guns are also brilliant solutions. I found this site looking for the British Sten-mk2, but I have not found. Can it be possible that you analyze? thanks.

  • Keith

    According to Chinn (The Machinegun Vol 4), closed bolt blowback would require a recoiling mass of around 380 pounds to prevent case separation.

    With the Oerlikon’s countersunk chamber and long bolt head, you might get away with a bit less weight, as the case remains supported for part of recoil, but the reduction would not be much.

    I don’t envy anyone trying to come up with a light bolt head that would feed and chamber the cartridge to the correct depth each time (allowing room for the case to continue forward as it ignites), ready for the main recoiling mass of 40 pounds or so and carrying the striker to run forward and fire it when the trigger is pulled.

    I also wouldn’t envy the task of re-working that trigger mechanism to semi auto only.

    Great site!

    Keep up the good work. I’m with Al on the all guns large and small theme.

  • Alan Hays

    The .pdf of the Oerlikon manual was digitized from the copy of the original manual I made. The original manual was borrowed from the Navy Yard Museum in Washington DC. I made my copy probably sometime around 1999.

  • John

    I have a complete bolt, cotter and trigger lock assembly I’d like to sell if anyone knows of anyone who might be interested. there is Exc. condition!!

  • larry

    I have a drum magazine for sale if any one is looking. Gray painted and say’s made in new jersey for export.

  • Ron MacKay, Jr.

    On US Navy ships during WWII, did 20mms have a mechanism to limit the barrel’s train and/or elevation so the gunner could not aim (or fire)into his own ship?
    Thanks!

    • Fred Maupin

      Depending on the mount, there were provisions for adjustable traversing stops in the form of steel lugs which limited the movement to 180 degrees outboard of the vessel

  • Hoppy Hopkins

    While the 20mm Oerlikon auto-cannon was one of the most effective 20’s of WWII and the PDF Manual is wonderful, I find it frustrating in the extreme that there appears to be no ready source for the blueprints and manual of arguably the best 20 of the war. I am speaking of the Lahti L-39 and its AA variant the L-39/44. I probably will never be able to afford an original, but it would be wonderful to be able to build a Non-firing full scale replica and as a machinist, I could do exactly that if I could find blueprints. I know that Red Jacket rebuilt one for their “Sons of Guns” TV show, but never knew if they had prints to work from. Does anyone have a solution or suggestion?

  • Dave

    On the twin mounts we have on the LCS-102, the barrels can spin 360 only if they are locked straight up.

  • Hoppy Hopkins

    As to your question whether we would like you to include heavier weapon systems along with light arms; the answer from me is an emphatic YES. There were a lot of unique designs available in the mid-range size artillery/AA weapons that could do with a bit of examination. The really heavy stuff, it another story though; once you have seen one interrupted thread breach, the Whelen (sic) stepped breach and the sliding breach, you have pretty much seen them all as they all are simply variants of the originals.
    The 20, 22, 25, 30, 37, and 40mm stuff on the other hand had a lot of variation. For example, the Lahti L-39 and the Soumi both were anti-tank rifles but both utilized very different approaches.

  • Joe Hughes

    I am a crew member on the Battleship IOWA Museum in San Pedro CA. Our ship had 3 careers (WWII, Korean War and the Cold War) and during her last major overhaul all of the antiaircraft weapons (40mm Bofors and 20mm Oerlikon) were removed.

    We like to tell our guests about the ship’s mission during all of her careers but all we have to show for the 1940s AAW capability is a couple of inert 20mm and 40mm rounds plus pictures. The pictures don’t do justice and we need a few guns for display.

    I am working with the Navy to get a 40mm Quad Bofors for display. However, finding a 20mm Oerlikon has been near impossible. If anyone has a source for a 20mm single barrel I am interested in buying.

    Thank you.

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