Close "Boyfriend Loophole" in Massachusetts and prohibit all domestic abusers from gun ownership, not just those who are "currently married to, formerly married to, has a child with, or lives with the victim."

https://www.dailykos.com/stories/2017/11/12/1714045/-The-deadly-link-between-domestic-violence-and-mass-shootings​

https://everytownresearch.org/reports/mass-shootings-analysis/