Guns and the Colorado Shooting
A very short and extremely thoughtful piece in the Atlantic unpacks some useful data on this question.  Well worth a read. 

While I see great value in the 2nd Amendment, I wonder about the claim that wider citizen gun ownership would have stopped that Colorado shooter.  First, gun ownership is already widespread.  Second, the claim can only mean two things...and neither meaning seems to hold water upon closer examination.

Either the shooter would have been stopped if most or all of the theater goers were armed...but this could only be a recipe for a more chaotic scene with many more deaths.  Or the shooter would have been stopped if one or two of the theater goers were armed and stood up for the rest of us...but isn't that what we call a police force?