A ten-day waiting period would have stopped the mass shooting in the city I work for. 18-year-old male walked into gun store and walked out with a new AR-15, several 30-round mags and four boxes of .223 ammo in a little over an hour. His intent all along was to shoot up a party attended by his ex-girlfriend. Less than a week after he bought the rifle, he deployed it and killed his ex and two of her friends.
He was not crazy, no mental health issues, honor student and not bullied. There were no red flags as far as law enforcement knew. He was immature and couldn't deal with rejection.
The difference was the AR-15. The most lethal weapon anybody can legally buy, and the favorite weapon of mass shooters looking to run up the score. It should be pulled from the market along with every other semi-automatic rifle. The only rifles that should be available for civilian use are bolt and lever action.