It is a homemade semi-automatic shotgun.

“Creator: 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.”

Video after the break.

[via Techeblog]