Since 1960, the United States has reduced the rate of deaths in automobile accidents by about four-fifths: from about five deaths per 100 million miles driven to about one.
This reduction in the casualties from driving was achieved by a complex of measures: cars were improved, new safety devices were introduced, road standards were raised, trauma medicine advanced, and tough measures against drunk driving introduced. People still die on the roads and always will, but we no longer suffer the carnage of the early 1960s.
When thinking about gun measures and mental health measures, the right question to ask isn't: will such-and-such a measure prevent all killings? The right question is: will it contribute to reducing the number of killings as we have previously successfully reduced automobile fatalities?