Calling people mentally impaired locked people away in the Soviet Union. Who defines mental illness? Gender dysphoria went from a mental disorder to not a mental disorder more because of political pressure than science. Should transpeople be denied their 2nd Amendment rights if the people who want it so obtain political power? No victim, no crime. There is no victim in the sale of a firearm. Potentially? I have a penis so I’m a potential rapist and if you have a vagina you are a potential prostitute. Do we want laws about what someone might potentially do? I don’t.
In London they’ve gone from gun control to knife control; put your kitchen knife in a bin to take the popular weapon out of circulation now that guns are banished. In answer to your question, never. There is nothing to do. I’m more likely to die slipping in the shower or being hit by a red light runner as I walk in a crosswalk.
If you are truly serious about preventing mass murder, disarm governments and their enforcers. None can compare to the murderous nature of government. I was once upon a time one of our government’s enforcers. As a Marine fresh back from Vietnam I was trained in riot control. Our instructor told us, “They are your fellow Americans. Use the minimum force necessary, but all that is necessary.” Sadly, at that point in my life I would have killed you if the government deemed it necessary. People take laws far to lightly.