"These tragedies must end," says President Obama, referring to Adam Lanza's horrifying assault on Sandy Hook Elementary School in Newtown, Connecticut, last Friday. Although it is hard to disagree with that sentiment, writes Senior Editor Jacob Sullum, the measures Obama favors cannot reasonably be expected to prevent such thankfully rare but nevertheless appalling outbursts of senseless violence. Indeed, we know for sure that an "assault weapon" ban would not have stopped Lanza or made his attack less deadly, because it didn't.