I'm sure a lot do. Killing sprees in the projects don't tend to be much publicised, because nobody cares about poor people shooting poor people. How many of the guns did the guy who shot in the cinema legally own? Or were they all stolen/brought from criminals? Even here some of the killings we see are from legit guns despite the gun controls we try and uphold.
All of his weapons, armor, and ammo were purchased legally. They knew within a day or two the exact stores they were purchased from, which I thought was pretty amazing. Not on a good way. It means that the database for gun ownership is far more exact and comprehensive than I thought it was.