Try listening to the "assault rifle in every home" arguments on the same day you read about the outlaws with machine guns who are running Venezuela while Hugo Chavez lies dying in Cuba. ("Slumlord" by Jon Lee Anderson in the January New Yorker). You may begin to imagine these fervent pro-gun crusaders marching up your driveway after some terrible calamity to claim your food and water for the righteous.
Its true that if a person stumbled into the vision of a post-apocalyptic film-maker or gun-maker, a machine gun would be handy. Our government is shady but I would still rather give the machine guns to a well-trained, publicly accountable militia (such as police and national guard) than a bunch of fearful, self-appointed people. Exactly who are they planning to protect in the case of social breakdown? They will become the roving bands of outlaws that they say they want to protect themselves from.
Do we need a well-appointed, democratically controlled local militia to protect us from the second-amendment militants?