What mature solutions have we developed so far? 30 pages, we've got to have something....
At a minimum:1. Ban assault weapons. The gun hobby guys want to fight about what this means, but I say any rifles or weapons that are automatic or semi-automatic and capable of mowing down innocents the way it happened last week. I'm sure the gun folks can come up with the right definition.
Beyond that:
2. I think you really need to look at banning handguns. They're always involved in these sorts of things. It's too easy to hide them and take them anywhere. Yes, some people will be bummed they can't have their target practice hobby anymore, but they can find another hobby, and if less people are killed, that's an OK trade off. The handgun ban in England is a good model, and gun homicides are breathtakingly low.
I can see some exceptions, because an outright ban would drive the far right bonkers. Hunting rifles seem relatively unproblematic. Leave them legal but regulated.
Try it out for 5 or 10 years. Let's see what happens.