[graeme]

Sorry, I should have said "cause of the difference"

Obviously people don't go on sprees merely because they have a weapon. But they need gun access in order to commit a mass shooting.

I do agree there might well be some additional cause. I think if you flooded Canada with guns you would see more mass shootings, but perhaps not as much. The US does have an overall higher crime rate, after all. And mass shootings do occur elsewhere, though the US is consistently high up per capita.