To prevent mass shootings would require the total destruction of all firearms in the U.S. whether for protection or sporting use. This lunatic used a shotgun to start his spree. There is no stopping anybody in most states from grabbing a sporting shotgun, entering a school or building, and slaughtering innocent citizens. It would be far better to incarcerate sick individuals like Alexis and keep them there. We can blame libs for most of the mass slaughters by firearms that have taken place in the last fifty years. And for most of the violence in the minority communities that the lib press blissfully ignores which far outnumbers the periodic, mass shootings in general.
Then some other instrument of death would be developed (Chemical?) to take its' place. Necessity is the "Mother of Invention."