Or have Mad Bull manufacture a shell that can have rubber or foam slugs also have the capability to accept their 5 or 6 shot C02 Shot shells, just keep the barrel 12ga size, you don't really need a full length barrel for the 6 or 5 shot co2 shot shells. Like you said they are for breeching and cqb engagement. also those mags on the Striker and the AA12 are HUGE, and after all Airsoft guns are supposed to be 1:1 replicas so why make the mag smaller. The trick would be getting the automatic system to be sensitive enough to blow back and eject the shell with the CO2 shells.
As seen in this video, this is what I do with my 870 it works really well, I know it probably isn't legal but I don't have $180 extra bucks to get a m500 gas shotgun. Since my 870 was given to me I didn't have to spend a crap load of money haha, and besides with slugs and even Double 00 that thing kicks worse than my bush wood tractor axle haha (K98 Turkish) And also yes I painted the tip orange can't be too safe.
http://www.youtube.com/watch?v=QX_UmtD7KP4CO2 powaed wemmingtin!