Short of confiscating firearms from people that can't assert they aren't crazy assholes (which will never happen), we would have to turn America into a series of fortresses.
That would suck.
Any place that wants to be "safe" has to be designed to maximize outward visibility, fortified, and staffed with sentries to watch for threats. Entrances would have to rigorously check people for weapons (with only trusted people being allowed to carry anything that could reasonably used as an effective weapon).
Sentries are especially important because they can actually notice threats. That is their job when they are on duty: to watch outward for threats. But sentries are expensive because you need them watching all directions, with excess to cover breaks, and all hours (otherwise a determined attacker could sneak in when there is nothing to guard and wait until the time of day there is something to guard).
So far that is just a lot of really costly expenses (buildings exteriors that stop bullets aren't cheap, nor are the salaries of sentries and guards).
But living in what are basically war fortifications is going to provoke a lot social regression (not to mention create a powder keg when a sentry mistakes something for a threat and a person is killed -- because that will definitely happen eventually). Lines will solidify and basically you get post WWII Berlin, but instead you have a lot of these factions that are ready to shoot at each other. Someone will bump someone else, and rumors of it will cause factions that consider each side a member will escalate things and the shooting starts.