It's interesting, if only because there really aren't many credible gun studies out there.... They have 2 big ones and they're both stupid.
The first, that says guns cause crime, doesn't consider you stopping a crime unless your shot at, or stabbed or something. So if someone came at you with a knife, or pulled out a gun on you it wouldn't be considered stopping a crime.
The second, counted stopping a crime anytime you felt threatened, so if you pulled a gun on someone who randomly came up on you in the ally no matter why he was walking up to you.