Virtually all mass shootings have involved legally owned firearms that somehow have made it into the hands of mentally unstable individuals. The only way to prevent the mentally ill from obtaining high-capacity assault weapons is to ban them from private ownership. This requires a repeal of the second amendment so that a set of sensible laws can be written to restrict their ownership, use and safe storage.
In the case of mass-shootings, it is not the criminal element that we have to worry about - it's the rest of us. This is precisely why restrictions on private ownership of assault weapons will work to stop these kinds of incidents.
We want to live in a society based on the rule of law and not on an apocalyptic vision of the future based on shootouts at the OK Corral.