@AthenasOwl Do assault weapons account for the difference?
I don't really understand the assault weapons ban, or at least the way it was written – I could see banning firearms altogether, or banning certain calibers, but who cares if a mass shooter has a barrel shroud and a flash suppressor or not?
I know some mass shootings used "assault weapons", but it seems like the shooter would have murdered just as many people without them in most cases. Different features, not covered by the ban, seem more important: Bump stocks are now banned, for example.
So... I'm thinking something else changed in the last 15 years to account for more deadly mass shootings. Media coverage seems to be another common answer, but that doesn't seem convincing either.
@ech @AthenasOwl I found this informative. High velocity rounds appear far more likely to kill.
@royal yeah, rifles vs. pistols is a meaningful distinction: rifle rounds can have a lot more energy. And like I said caliber. But I don't think AR-15s are any more deadly if they have barrel shrouds, pistol grips, and flash suppressors on them. 
@ech I agree. Massive proliferation starting in the late '90s and accelerating about 2008 is likely a major factor. By 2009, guns outnumbered people in the US, and are now 20% higher. A downward trend in gun homicides also ended around then.