Around 50 million Americans have a diagnosed mental health issue. If we were actively working to accurately diagnose everyone (and remove the stigma to encourage people to be diagnosed) that number would soar. To accurately gather and track all this info would be an effort along the lines of implementing Obamacare. Then there's the infosec nightmare of trying to protect that information. All of that is before we touch on the logistical nightmare of using this information in a meaningful way.

It could be done. All we'd have to do is empower the federal government to provide mandatory mental health screening for 330 million Americans (ideally along with free/low-cost ongoing care for everyone with a diagnosis). Then you have to empower law enforcement to closely monitor everyone who fits the risk profile. We know what that profile is - male, mental health diagnosis, history of violence against women. That could describe a shit-ton of Americans.

The deadliest mass shooter in American history is Stephen Paddock, who wasn't some asshole kid telling his friends on YouTube he was going to shoot up his school. But he did have a history of anxiety and depression, a gambling problem, and was known to be an asshole to his girlfriend. You want the FBI showing up every week for spot checks at the house of everyone who fits *those* criteria?

Yes, we have a mental health problem, and addressing it would almost certainly make a real dent in our mass gun violence. But the people claiming it as the key to solving gun violence would never allow anything close to a real solution.