If the US is unwilling to restrict the sale of automatic assault style weapons to protect their citizens from nut-bars than a predictable solution is to make use of the vast military personal they already employ and post them at every school, church, shopping mall and any other location that the public gathers in quantity. Most Americans would hate to see this sign of force on their streets but the reality of the situation is that it is will only get worse with no movement towards common sense.
It's not uncommon in many countries such as Turkey, Egypt and S. American hot spots to see military personal stationed with assault weapons scattered throughout populated areas ready to engage. It would certainly make a psychotic killer think twice if he knew he would not attain a hefty body count to immortalize his worthless life when selecting a soft target.