I have partly modeled FN303 mags from another project I was working on.
I will put the files up later. If someone want to borrow a mag to refine the files further then I can mail one out.
We would need to move the magazine up from the image shown so the bolt axis lines up with the mag opening. But I am ok with that.
Making this from a mini or axe may work very well. Cheap commonly available platform. We would need to lose the breech eyes and make some board layout changes. Longer bolt to cross the mag too.
I'd take a 12g change on the Asa or small air tank as a compromise to how we power it. Heck then you could even put a remote nipple on it and run it that way.
Double fisting these would be mean!