I propose that the government make it mandatory for all private gun owners to submit to an online test every six months that:
1. Checks if the person is mentally fit using questions from standard psychiatric tools such as depression screening tests.
2. Checks if the person has basic knowledge about using and maintaining guns, and knows how to properly store guns out of the reach of children.
3. Checks vision: a simple target shooting type game where targets are either deer or innocent people and the objective is to save innocents and shoot the deer.
4. Educates about gun safety
Failing the test should result in temporary impounding of all firearms owned by the person.
This will work because most mass-murders will not be able to pass even a basic psychiatric evaluation.