It stems from the liberal argument dichotomy.
Out of one side of their mouths they claim a citizen with a gun will only spray bullets and endanger everyone save for the criminal that they are shooting at - as if we were all NYPD officers trying to shoot a guy 20ft away on the sidewalk.
From the other side of their mouths, they will argue that it only takes one shot per attacker, because everyone knows how to shoot and where to aim to instantly kill 5 or more bad guys, so no one would need a magazine holding more than 2 rounds. A woman actually argued this at a town hall meeting last week, she was one of those people you look at and you can see crazy oozing from her pores like Jared Loughner - perhaps it was his mom.
When the whole idea is brought up from either aspect, challenge them on this contradiction in positions, because they will be at a loss to explain it and likely will slip with their real belief that no one should own a gun. At that point you can label them crazy as its been made patently obvious that citizens with firearms have stopped numerous crimes. It is always good to have a few examples to cite of that as well that you can repeat when they make the absurd assertion that no one ever defends themselves with a gun.