There are complicating factors when looking at this phenomenon. For one thing, a mass shooting is only one form of mass killing. Bombs, knives, axes and motor vehicles outshine guns as the preferred means of bumping off people in various countries, and comparisons based purely on incidents of mass shootings may be misleading.
For another, there are different contexts in which mass shootings occur – family blood feuds, rival gang shoot-outs, terrorist hit-squads, and lone killers opening up on people at random all present very different scenarios, and there is a strong case to be made for separating some of these categories out – terrorist attacks by trained operatives, for instance. But for now I’ll focus on all situations where guns are used to kill a number of people.