Watch the video and notice how plastic pellets simply tear up the target into confetti. Your flesh ain't exactly hardened material and I have a feeling getting shot with one of those could be a painful, if not lethal experience.
And thus, we come back full circle to the stupid idea that you can ban an object and somehow change people's behavior. The knowledge on how to make a tool that throws an object at high velocity is now commodity, illustrated by the fact This guy built this for his own personal entertainment. Imagine what the market would be like for someone with skills like that if it were more difficult to get a hold of mass produced firearms.
You think that driving the firearms manufacturing underground will still give you serialized, traceable firearms?