By defender type, do you mean with a pistol grip only? My limitted knowledge tells me, that most semi shotguns, have a spring extending back into the stock, making such a thing difficult at least. The Saiga Shotguns don't have this problem, if you don't mind moving to the USA or New Zealand. The only Mag fed semi, that I'm currently aware that would be legal here, if it existed, is the mag fed Cougar that doesn't even have an ETA yet, and with the spring, a Pistol grip only stock, would be a no go.
Of course, what I don't know could fill volumes, so maybe someone else knows something I don't.