I'm a bit tired so unable to engage too much but I will throw out a few half baked ideas.
1) Pass (and enforce) laws that require legal gun owners to keep their guns from falling into the wrong hands. This means no leaving guns in vehicles, unlocked cars or other places where they can be easily stolen, and locked up in a manner that savvy teens can't get to them either.. Biometric safes and locking devices are well within the realm of current technology. If my truck can open its door when a touch it, we can certainly find ways to use that to keep guns out of unauthorized hands.
Sure, that means there are still many, many guns out there in the hands of legal gun owners, but that would knock out SO many of the guns used in crimes. I am fully aware that mass shootings often involve legally purchased guns, however, mass shootings are a very small percentage of homicides overall, so that is not my focus here.
2) Decriminalize cannabis nationwide, and contemplate other changes in the way we fight the "war on drugs". The majority of homicides are tied to the illegal drug trade, and/or the culture of violence that springs from it. We need to "cancel" "artists" who glamorize drugs, misogyny and violence, and perpetuate attitudes of hopelessness and victimhood. We need to boost the influence of positive role models who show kids the path to success and empowerment is through education or learning a trade, not becoming a rap star or professional athlete.
3) Face misogyny head on. SO many homicides trace back to a simple problem. A man, who believes he is entitled to the attention (if not outright ownership) of a woman. If said man is rejected by a woman, he seeks revenge on the unjust world by killing said woman, or her family, or a man who looked at her in a bar, or simply random strangers. This is a common thread running between mass shooters , domestic violence killings, bar parking lot massacres, AND even the drug related homicides.
I don't have all the stats in front of me and I'm not going to bicker over numbers. I sat across the table from HUNDREDS of killers, even more victims families, talked with them, and this is what I learned.