You could have civilian access to every military gun but if the population was law abiding, the rate of crime would not be affected. People who want to commit violent crime will use whatever means are at their disposal. The answer to reducing violent crime is the removal of the criminal people. That is why the crime rate, while still much higher than it was 50 years ago, is constantly going down. Offenders are getting locked up in prisons at a higher rate than new ones are appearing.
The way to stop mass shootings is to improve mental health services and remove the stigma associated with being depressed. The destructive policies which are leading to community breakup must also be reversed.