I have no problem with background checks that involve criminal checks. mental health screenings, and other (radical Islam, etc.). The overwhelming majority of guns are sold this way (78-86% of all guns).
Most of these shootings would not have been prevented with this kind of screening.
Yes I agree stronger gun laws would not have prevented MOST of these shootings, but if it would have prevented SOME of them it would have been worthwhile to enact stronger laws in this regard.
Tom K