^ Mass Shooting Tracker counts any shooting of 4 or more people as a "mass shooting", it doesn't have to be in a public place nor are the victims necessarily innocents. The below addresses what more of us consider a mass shooting -

power of prayerful repetition

two Supreme court clerks who worked on "Heller" for the involved Justices on either side of the 2nd Amendment debate warn everyone that not a single Justice felt the 2nd Amendment was against some kinds of gun regulation

excerpt

[....] Most of the obstacles to gun regulations are political and policy based, not legal; it’s laws that never get enacted, rather than ones that are struck down, because of an unduly expansive reading of Heller. We are aware of no evidence that any perpetrator of a mass shooting was able to obtain a firearm because of a law struck down under Heller. But Heller looms over most debates about gun regulation, and it often serves as a useful foil for those who would like to deflect responsibility — either for their policy choice to oppose a particular gun regulation proposal or for their failure to convince their fellow legislators and citizens that the proposal should be enacted.

The closest we’ve come to major new federal gun regulation in recent years came in the post-Sandy Hook effort to create expanded background checks. The most common reason offered by opponents of that legislation? That it would violate the Second Amendment. But that’s just not supported by the Supreme Court’s interpretation of the amendment in Heller. If opponents of background checks for firearm sales believe that such requirements are unlikely to reduce violence while imposing unwarranted burdens on lawful gun owners, they should make that case openly, not rest on a mistaken view of Heller.

Justices don’t control the way their writings are interpreted by later courts and other institutions; certainly law clerks don’t. So we’re not asserting that our views on Heller are in any way authoritative. But we know the opinions in the case inside and out [....]

a lay person's summary of what they say:what even "originalists" feel is protected in the Amendment is like a right to keep like a handgun for self-defense in the home, otherwise, everything else is fair game for regulation!!! All the spin beyond that is bogus, none of the Supremes agree with that.

Ohio governor signs law allowing teachers, school staff to carry guns with 24 hours of training