Oh I know that most mentally ill people don't shoot up schools. I have borderline personality disorder myself.
But I'd say 99.9% of mass shooters are mentally ill. That was my point.
Going by the linked report, it's closer to 66% with a documented history, or about two thirds (which sounds higher than it is -- the
base rate for the US population is around 50% for developing a history of it at some point in their life, so it's higher than the general population for the shooters in question, but not wildly so). Even if identification rates got better, I doubt it'd get much higher -- a lot of mass shootings are spur of the moment things, which don't take having lingering psyche troubles to occur. The issue they have is easy access to a particularly deadly weapon, full stop.
Extremely pointedly, though, the number of cases where it's
directly linked to a symptom of mental illness (i.e. hallucination, delusion, etc.) is closer to a 6th or so -- only 16-ish percent. It's not mental illness causing this stuff. Correlating, but not causing.
... though so far as practicality goes, you'll note I already chimed in on that. There's nothing practical that
can be done stateside, with the way things currently are. The precondition to any meaningful improvement on the gun violence front is to break the american right wing as a substantive political force, because without that you're not going to accomplish much of anything with them actively attacking every possible avenue of improvement, even the goddamn ones their own politicos are suggesting. Until their horseshit can be stopped, fuckall is going to occur on this front.