I suppose people make AR uppers and lowers out of anything these days, but do you think making an AR upper out of scrap iron would be possible? I've heard of ductile iron and its many uses in things like decorative drainage grates, car parts, and pipes. Obviously, it would lack the light weight and corrosion resistance of Aluminum, and would probably begin rusting the moment it's pulled out of a mold. But, I suppose it would be durable enough to be machined but I think it would be an interesting project for someone with the gumption and the metal casting equipment.
While I suppose that Aluminum has a lower melting point, I'm unsure how well a home-cast aluminum upper would turn out.
I could imagine someone also touting the rifles as "Green" since they would be using recycled iron that would otherwise be going to waste somewhere else.