I think this is an often overlooked point, and a correct one. There have been millions of firearms in the country for friggin' ever. And for most of that time in most of the country it has been fairly easy to obtain a firearm, both legally AND illegally. As well, since the country has existed, virtually every firearm has not been used in a criminal manner. My guess is that 99% of them have never been fired at another human in anger.
But something has changed over the years as we are seeing more and more of these mass shootings (almost however one chooses to define them). A change in the conditions usually warrants a change in the response. I doubt we will ever see it. These kinds of mass killings, whether they are pure political terrorism, hate crimes, or just some psychopathic nutjob, are here to stay. They are the new normal. My guess is before the presidential election, there will be at another. And another after that one. And another one after that. As a people we can do better if the political will was there and the leaders of this nation -- on both sides of the aisle and the argument -- had the courage. I'm not holding my breath.