I think that, again, ignores the elephant in the room. None of these shooters give a shit about laws, licenses, or regulations. Psychotic people are incapable of that type of thought. Even people who are not technically psychotic, just really pissed off mood disordered people, have no sense of self control. They are without impulse control. Unless you can catch them in a very narrow window of time you can not stop them.
I'd be interested to know what these other countries, who have it under control, have for psychiatric care. Serious question. I'm pretty fatalistic about it. I'm not a gun guy, so I don't care if we take them all away, but I just think that is treating the symptom. The disease is mental illness. Not frequently, but several times in my old mental health career, I was charged by a florid schizophrenic. I was lucky enough to have copious back up, but a raging schizophrenic will fuck you up.