It doesn't have to make the whole gun, just the frame, which is considered a 'firearm', legally. People have been 3D printing those for years. They are the "plastic guns" the grabbers have been having fits over, even though the barrel, springs, and trigger group, and at least parts of the slide are made of metal. Even without all those parts, the frame is still legally a firearm.
So you mean, this fellow produces a lower (I guess it is called, I could be wrong) with no trigger ass’y, no barrel, no slide, no spring, and a normal beat cop doing weekend buyback duty, sitting at a folding banquet table in a parking lot identifies this as a gun and hands over the reward?