How many of those shooters were felons? Most legally bought their firearms. How many of them had prior mental problems and were treated for it? It's my understanding that a background check does not include information on mental disorder. THAT information needs to be included in a background check.
the problem with that is that "mental disorder" is subjective to another persons perspective. mainly a psych dr. and most of them are loonies themselves.
though not many here will admit it Huntnshoot is 100% correct in his premise.