Anyway, incarceration sends these MILLIONS of people to the best crime school in the world. Among other things, once somebody gets out of prison, they know EXACTLY where to go, and how to procure an illegal gun, no matter how restricted they are. If somebody has their mind set on doing a massacre, they're not going to worry about the technicalities of illegality for firing a weapon that they wouldn't be allowed to use, because they'll have much bigger fish to fry after the massacre - if indeed they survive, rather than dying from their own suicide or suicide-by-cop.
Mental health care is unquestionably important, and I've already said so in some other posts in this Forum and HT. Having a widely available and efdfective mental health care system, which is also affordable to the general population, will not stop all of the massacres, but I think it would stop at least one-half of them. The mentally ill person needs to have access to the care, as well as not quitting meds long enough for his/her mind to trainwreck, etc.
A good mental health infrastructure would do a HELL of a lot more for the quality of life in the United States, over and above "just" stopping most of the massacres.
Indeed, Republicans to HATE the possibility of a health care system for mental problems, because the model cannot easily be made into something that is actually insanely (sorry, but the word implies) PROFITABLE. Better to just let people's minds go entirely off the rails, because such services will never help create more billionaires, which is one of the main purposes for that Party.