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”):
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?