How do mass shooters choose their weapons?

The Pittsburgh Post-Gazette has a useful article breaking down the vocabulary around guns. They checked with some expert too:

“We’re all trying to understand something that is not within our capacity to comprehend. ... I can’t get in the mind of a killer. … They don’t seem to be choosing [weapons] that would cause the most damage. For the most part they’re using the most popular guns in America.” - Mike Bazinet, spokesman for the National Shooting Sports Foundation trade association

“They’re light, easy to use and easy to carry. ... They’re a civilian version of a military weapon — a lot of men want to have that.” - Shira Goodman, executive director of CeaseFire PA