What concrete is to be done? The usual response is: "Let's ban guns." But when I was 12 and still when I was 30, just about everybody had guns, myself included. The changes have to do with people, not guns.
I should preface this by stating that responses to these mass shootings are of course emotional. These types of shootings represent a tiny fraction of American gun violence, but their random nature scares people, so the emotional response is understandable. That is, if somebody next door shoots his wife or brother-in-law in an argument, it's easy for most people to say, 'Well, he hated his brother-in-law, not me, so who cares?' This is an admittedly crude way to put it, but it's not a bad summary of what happens, imo.
You ask 'What is to be done?' Tough question, but it must be pretty obvious by now that doing nothing isn't working very well.
The aphorism "guns don't kill people, people kill people" is more a signal of tribal affiliation than explanation or analysis. But accepting it at face value as the cause of shootings, wouldn't it be in your collective best interest then to at least be more careful about which people are allowed to buy guns? There are exams to pass before you're allowed to drive. There are exams to pass before a HAM radio operator is allowed to transmit.
Why is there such a frenzied reaction by the gun fetish lobby when someone suggests something along those lines when it comes to firearms ownership? Is it REALLY a violation of your constitutional rights if you have to answer some questions and wait a week to be allowed to buy a gun to "defend your family"? Isn't that argument a little thin?
The thing is, as you and others have often pointed out, there already are a lot of guns around and it's silly to try to curb their proliferation at this point. There's a lot in that, it's difficult to argue with and I won't try. Because of that, it somehow follows that the "good guys" need guns too to defend themselves. But isn't it kind of the case that any "good guys" who were going to buy guns would have done so by now? Isn't it the case that a lot of good guys have guns now already? Wouldn't it be nice for your belief system if there were at least one or two examples of one or two of those good guys actually being present and stopping one of these mass shootings? I mean, shouldn't there have been a few cases by now if it was true that only a "good guy with a gun can stop a bad guy with a gun."
But as Rob C stated, the status quo is stalemate and likely to remain so. This is too sad for words.