Plenty of mass school (or other) shootings were done by legal firearms, using legally bought ammo.

I get what you say, but if folks are smart they should be concerned by any firearm, legal or not. Its trivial in US to obtain one if your record is still clean, you can be a proper psycho weirdo and still get it.

The people afraid of doing psychotests to get a gun which can kill tens of people easily in skilled hands... I'd say they are afraid for a good reason. Its like being afraid of driving test to get the license to drive.