As far as legislation to screen for mentally unstable people is one of the best courses of action. However it would have to be nation wide and most likely Federal, and good luck selling that to 2A extremists who will fight tooth and nail over the fact that they have to go through another step before getting their rifle. I’ve already tried it with some coworkers. 90%+ of gun owners agree that mental cases shouldnt have guns, yet they don’t agree with the fact that they will have to be screened and follow the rule too.
Chemical/biological/explosive argument is extreme, but it is so for a reason. With 300 million plus people in this country, the most extreme cases become likely and realistic purely from statistical standpoint. .00000033% of probability means there is a person out there right NOW that falls into that statistical average.