A while back there were .410 semi-auto AR shotguns floating around. I thought they were neat, but at the time I wished they came in 12 gauge instead. It looks like my wish came true! However, only 5-round magazines are available because they are imported from Turkey. A little pricey at $700, but neat. MKA 1919 Semi-Auto 12-gauge shotgun:

http://centerfiresystems.com/MKA-1919.aspx