Here in Baltimore, 99% of these "mass shootings" are gang bangers. The law wouldn't affect the "mass shooting" rate at all. The Parkland shooter broke the law, made terroristic threats on social media, which is already against the law, and L.E., the F.B.I. specifically did nothing. How many of these "mass shooters" are NRA members? Has there ever been a mass shooting at a gun show? These animals prefer unarmed victims. The worst "mass shooting" in Texas was the "Luby's Massacre". That coward had handguns, killed 23 people. He walked around the restaurant for 12 minutes, grabbing people from under tables and killing them. Suzanna Hupp left her gun in her car. She wished she hadn't. Here is her testimony before congress.
http://www.amazon.com/Lubys-Legislature-Womans-Against-Control/dp/096567844X My suggestion would be to make the whole country a "shall issue" country. Armed citizens would've changed the outcome of all those shootings. There are already schools that have armed teachers. And they advertise that. Remember, given the choice, these killers prefer a soft target. Also, arm teachers. The only thing that hasn't been tried yet, is the thing that would have the biggest impact on the high body counts. "Red flag laws" just makes it easier to disarm the public.