We tend to respond in one of two ways to the news that a deranged gunman has fatally shot a group of people with an extraordinarily lethal weapon. Some people are simply horrified. Others wonder where they can buy one too.It is widely understood that, when it comes to guns, bad news often makes sales go up. So it should come as little surprise that the FBI reported a surge in Glock handgun sales in Arizona following the shooting rampage in Tucson that killed six people and wounded 14, including...