I would say you have a lot to work out. Here's some things to think about.
1. The one thing you have going for you is the AA12 is so plain *cough*ugly*cough* looking if you have some knowledge of fabrication you shouldn't have to much of a problem making a body that looks like it.
2. You may want to look at existing airsoft shotguns and try to figure out a hack instead of an entirely new platform. Basically take something and make it look like an AA12. Something cheap like this
http://www.airsoftgi.com/product_info.p ... ts_id=5903 .
3. It's gonna be next to impossible to make a fully automatic airsoft shotgun from scratch, with limited knowledge, and without a good machine shop. To my knowledge there isn't even a semi automatic airsoft shotgun out that you could reference (I could be wrong).
4. In theory as a skirmish gun, an AA12 airsoft gun would be something i wouldn't want to take on. But as everyone else has said, picking up your $20+ a pop gas shell (that probably wouldn't work very well after they landed in the dirt) is something you don't wanna do. Nothing annoys me more than some idiot who purposely drops their airsoft magazines during reloading (p.s. if that's you its called a dump pouch and some practice folks), then spends an hour between games trying to find them. Think about 20 or 30 little shells scatted where ever you shoot. Unless you wanna walk around with a stupid looking shell catcher ruining the feng shui of the gun you spent all that time designing and building.
If i was you, honestly i would wait. I'm pretty sure it won't be long till there is an AA12 on the market. As MLA so softly put it "You just seem like some kid that fell in love with MW2 and made this post after your first round."... does make a point. It is a MW2 gun, it was on future weapons, so that means people will want one for airsoft.