This is awesome:

Derek writes:

I have designed and patents pending on this.. Its a beltfed 12 gauge upper receiver for a ar-15 or m16 lower receiver. It uses your lower receiver, so if you have a ar15 its a semi auto shotgun, however if you have a registered m16 lower or a drop in auto sear it becomes a full auto 12 gauge machine gun. This is a prototype model and all the bugs arent worked out quite yet. Most of the problems that I have encountered revolve around the shotgun sheels not being consistent enough to eject sometimes.

I will be doing a brief interview with the Derek soon.

Hat Tip: Say Uncle