"I have no knowledge or understanding of what i'm doing but I want this custom design that nobody knows of or has done before to be the cheapest of the cheapest, cheapest, cheap, cheapest, of the cheapest."
Your best option imo is to get someone to 3D model that clamshell uzi and design it in a way to fit an aeg gearbox inside. And none of that is going to be cheap. Why not just make the clamshell and do some special effects? What is even the point of it firing BBs? You'll just make it look like some 2008 youtube nerf war.