At first I pondered this relatively simple idea, based on the concept I used in my cartridge fed pneumatic, the same idea but eliminating the cartridge, the concept looked something like this:

After looking at this design however, I thought I'd make something similar scaled down 6mm calibre, currently making a prototype (photos to follow as construction proceeds). i'm building it as per the animation attached below. The hope is that with a constant flow of air, the bolt will continue to cycle to give fully automatic fire. Ideally, one would be able to fine tune spring strength in order to achieve this, but once the prototype is finished it will be all sealed up, so I'm hoping to be able to vary the flow of air into the system to tweak it. If not, yet I still have it functioning for one shot per burst of air, I suppose I'll just have to find a way to "pulse" the air from my chamber.