The 2nd amendment gets way too much respect (really, we should all have access to nuclear weapons?) but there is one argument in favor of it which, above all others, displays the incoherence of the gun lobby's thinking.
The first failing of the argument is that it makes a claim which is fundamentally untrue. In my world of philosophical enquiry, that is a problem.
Incredibly, though, the argument has an even bigger problem: if the claim were, in fact, true, the argument would no longer support the 2nd amendment.
The argument is simple enough:
Banning guns would not have helped, since the perpetrator could just as easily have killed his victim with a knife, a club, a bludgeon or some other instrument.
The claim, of course, is false. The whole point of guns is that they make killing people easier. It is harder to close in on a victim and stab or club him, especially if he resists, than to point and shoot. If it were truly just as easy to kill in these other ways, why bother shelling out for an expensive gun when a cheaper two by four will do the job?
Still, the claim is made (mostly unchallenged) whenever proposals are made to regulate guns. And yet, the falsity of the claim is not even the argument's biggest weakness.
Consider a world in which the claim is actually true, where it really is "just as easy" to kill someone with a knife, a baseball bat or any other implement as with a gun. In that world, why would the people need to be armed?
If it truly is just as easy to resist the hated authorities without a gun as with it, ipso facto you don't need guns at all. QED. I hope against hope that I never see or hear the argument made again.