The regulated part of an AR-15 is just the lower receiver – the part with the trigger and the magazine well, and with the serial number. The rest – upper receiver, barrel, stock – is just parts subject to less government regulation than a double cheeseburger with a large soda. So to buy a lower receiver, I need all the forms and permissions required for a functioning rifle. Then I can get parts and build for myself whatever I want on top of that lower receiver, as long as the resulting gun is not something illegal for me to own, like a machine gun.
In fact, a skilled metal worker and machinist could make his own lower receiver from scratch. He could make one for himself, but couldn’t manufacture them for sale without getting a slew of permits and paying big bucks. But between a piece of steel and a finished lower receiver, is an eighty-percent-finished lower receiver. This unregulated piece of metal is not yet a lower receiver, so it can be made and sold by anyone to anyone. And then the buyers could get together at a local machine shop and have a build party.
With cnc milling machines getting cheap, it might be more effective to regulate ammunition than guns. But maybe people have already thought about that.
UPDATE 15 June 2013: It says here that the Santa Monica shooter’s AR-15 was built from an 80% lower. To close that loophole they’ll need to ban metal-working tools, and I wouldn’t put it past them to try.