They don’t want to reduce gun violence, they want to use it as an excuse to take guns from law abiding citizens.
Go after the criminals that have guns. If you are a felon caught with a gun make it a automatic 10 years with no parole for just having a gun.
The police go after people with guns all of the time. The short sightedness of your argument is that we need to wait for these people to become felons before we can act. That doesn’t help protect against mass shootings as every one of them had little to no prior criminal record.