That's part of the intangible. There are local stories where the presence of a firearm makes the bad guy rethink his plans, no shots fired, no crime committed. The only quantifiable is the mass shooter who goes through with his plans unmolested. Everything else is speculation one way or the other, and both sides hype the stories in favor of their viewpoint - and the truth probably lies somewhere in the middle most of the time.

And who's to say a 6-shooter is sufficient? Real life is not like the movies, where the incidental bad guy goes down with a single shot and hit (ok the main bad guy usually just keeps on coming smile ). Folks miss, even marksmen who spend hours per day on the range, and people can survive multiple gunshot wounds, especially if they're hyped up on andrenalin (or pcp). And while it might be reasonable to question who needs 30 shot magazines, the same could be asked of a Ferrari owner, who needs a car that can hit 180mph? Does firearm ownership always have to come down to need?
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I used to think it was terrible that life was unfair. Then I thought what if life were fair and all of the terrible things that happen came because we really deserved them? Now I take comfort in the general unfairness and hostility of the universe.