Water bottle rockets are popular in science classes and very easy to make. To be honest, I think that many people considered making one of these a long time ago(at least I did), but didin't due to the risk. 240g at 20m/s gives about 96J, close to a rimfire .22 short(aboout 110J). At 40m/s it is four times more, enough to cause seriuos trauma, even with a larger area of impact (nose cone). But what I am really concerned about are the fins.
I mean, really, with all the safety talk considering launchers, all the precautions taken by launcher builders, all the soft foam and nerf ammo, and now they come up with this? It's like taking a real sword to a wooden sword sparring match.