Well it's the homicide rate I think, which includes any killing by firearm including suicide. I wish we could detect if someone was purchasing a firearm with the express intent for suicide. Shopkeepers generally can because the customer will not express any preferences or have done much research and excuse certain telltale behaviors but you can get around it

Is the suicide rate going up or down with the murder (not homicide rate). That could tell us some things.

Additionally one could always commit suicide with their weapon they got when they were fine and have that count as a homicide and a suicide.