this argument does not have much evidence backing it up, for one, there is not much data showing that gun toting citizens have STOPPED a mass shooter.

and for two, there is also far less correlation to show shooters pick an area due to carry laws, or that they even KNOW the carry laws, but there is a reason to believe they KNOW how much damage the weapon can do when they purchase it.

And yes, a trained gun owner may eventually stop a shooter, and decrease the number they can kill, or they can, in the confusion, be mistaken as the shooter and shot themself ... but I think its more sensible to take precautions to prevent it even GETTING to that point as often ...