Are these things double stack? There is a 9mm SMG (Spectre M4) that uses a quad stack magazine! I'd love to see something like that in 12 gauge for the saiga. I have no idea how you get something like that to feed reliably, but i bet you could make a nice short 20+ round box magazine if you could perfect it.
http://world.guns.ru/smg/smg21-e.htm