That wld work if the person were currently "in a state" or clinically mentally ill. But there are many instances where "they just snapped," w/o previous indications. They may have been perfectly stable when they purchased a gun. As much as I despise semi-auto type weapons, I know ->

some use them for little more than target shooting & protecting livestock, etc. They like them. But no one knows what (or when) will split that thread between rational thinking and -not.

Australians own guns Australia does not have mass school shootings The only people that need automatic weapons are the military/police