Well, this forum for starters. I went through GD, P&E, IMHO, MPSIMS and the PIT just now and copied a link to every thread dealing in some way with mass shootings, and here is how many we have had:
I see zero threads from any of those 5 forums with activity in the past month, that appears (at least based on thread title) to be concerned with America’s general homicide problem (which in 2020, meant 13,500 gun homicides, of which around 450 were in mass shootings.)