Actually guys....
I have been big into airsoft for a number of years, and they have these new rifles coming out lately that use a gas blowback (GBB) mechanism. These GBB M4s have internal parts that are almost identical to a real M4. Now the airsoft upper wouldn't be able to fire real ammunition, but I am wondering if it is possible that a real AR upper could be modified to fit the airsoft lower receiver with a full auto control group... Watch the following vid, and tell me it doesn't look just like a real M4 lower...
The relevant part is at 0:42
http://www.youtube.com/watch?v=yh7xOJM_zss
Now this news story apparently happened a year ago, so I'm thinking that if there were any real concerns with a conversion, we'd all know about it by now...