Last year Mother Jones collected a bunch of data on the specific guns used in mass shootings over the past 30 years, from 1984 to present. The Washington Post took that data, added the past year's tragic statistics, and turned it into an infographic.
It can be helpful to look at graphics like this to pick out the larger trends, and also, on a base level, to see just how depressing it is that a mere three decades can produce enough data to make such an extensive interactive infographic. The graphic shows that the majority of guns were purchased legally, that many involved two or more different types of weapons, that an increasing number of shooters are considered to have a mental health issue, and that women do not really do this (only two instances overall).
Check out the graphic here.