"For a straight blow-back operated shotgun to work safely, it would have to operate from the open-bolt position with advanced primer ignition"
Actually, I was referring more to the 100+lb bolt body of the Oerlikon than its operation
The purpose of the API setup was not to reduce recoil (it's a fixed gun) but to reduce the mass of the reciprocating bolt. If fired from closed bolt, the 20mm rounds would require a 200-something pound bolt to safely fire, which would make the gun even bigger, and slow its rate of fire terribly (bad news for AA guns). As would be the case with a closed-bolt Oerlikon, you simply need to design your machine to have a heavy enough bolt (10lb or so)
Blowback is the most reliable way to go for duration-fire (or barring that, delayed blowback) because there are fewer parts to be battered (recoil action) and its tremendous fouling tolerance (gas op). I think plastic shotshells are slippery enough that they wouldn't grab the chamber walls under pressure and tear like brass does, either (not much of an issue in low pressure cartridges, but still). Since there's no carriers or gas tubes, it's way easier to make a ribbed barrel and fit a water jacket for cooling.
As I said, a 12ga closed bolt blowback would require a 10lb mass (bolt plus hammer/striker weight). Totally untenable for a carried weapon, but on a fixed platform, that may be acceptable, as well as very simple to build. You also have the benefit of 12ga being pretty low pressure, which makes it that much less likely a case failure would be catastrophic should the breech open too soon. You could easily find a big hunk of steel, drill it for a firing pin/striker setup, weld up a beefy receiver of steel plate or tubing, a dash of trigger group and machine work, some blood sweat and tears, and you'd be all set
If you don't mind my nosey-ness, what's this for? Just for fun, or do you have a reality show or something? (I kid, this thing just seems right up Red Jacket's alley is all
). Honestly, a minitature Oerlikon or similarly bad-ass anti-aircraft gun that runs on (much cheaper) shot shells would be a pretty cool toy
TCB