Two weeks after 19-year-old Nikolas Cruz killed 17 people at his former high school in Parkland, Fla., Walmart announced that it would raise the age requirement for buying firearms and ammunition at its stores from 18 to 21. The change will affect only sporting rifles and shotguns; the retail giant, which is the nation’s largest gun seller, has not sold military-style assault rifles since 2015 or handguns since 1993. While any retailer’s voluntary restriction on gun sales is welcomed by gun control advocates, evidence suggests that the new policy will not result in a significant reduction in homicides.