But it's interesting how the majority (there's always an exception) of mass shootings occur in places where guns are not allowed.
I don't think you have proof of this claim. I am fairly confident that most mass shootings do not occur in gun free zones. They most often occur in non-public areas and garner little press coverage. They actually happen quite regularly and don't even get much play in the media or here.
Turns out, a lot are familial and domestic violence, often occurring in homes of gun owners. We don't discuss them very much. Heck, most of us don't expect our neighbor, father, uncle, mother, etc. to all of a sudden try to kill us all, but if they do, it tends to be somewhat localized and often much worse of than this failed mass shooting at the Oregon mall (usually defined and 4 or more shootees in a given even or event series, not including the shooter).