I don't think so. Any projectile of decent mass will be deadly when accelerated to high speed. There's nothing that magically exempts potatoes from this just because they're edible. You can absolutely kill someone with a potato gun. The prosecution in question may be wrong for other reasons (e.g. fired safely and not with any intent to harm), but "potatoes can't kill, because otherwise Ireland would be a superpower" is not the way to argue it.
A comment was made about a lawyer making an argument which, in my opinion, is a bad argument. I pointed that out. I'm not being "pedantic" nor am I "nitpicking" when I point out that a potato cannon is, in fact, a deadly weapon and the lawyer's argument against it is stupid.
Now, I agree that the comment, and therefore the whole thread that followed it, was not really on-topic. But I'm not seeing this whole "pedantic and nitpicky" thing.
In addition to mikeash's correction, a round does not contain everything that you need. Without a chamber, a bullet is not going anywhere at a particularly dangerous speed. The casing is really not meant to contain the pressure of the burning propellant by itself and without a barrel then the expanding gases from the burning propellant would primarily dump their energy into the environment, not the bullet. In fact, without a chamber and barrel, the casing is actually more dangerous than the bullet, though still would not be deadly. This makes sense when you consider that the casing weights far less than the bullet.
