Wish I had the link to cite, sorry about that, but I read somewhere that that 355 number had been investigated and only 20 or so (21 I think) of those incidents even met the FBI's own definition of a mass public shooting. The rest were gang-related, some not even firearm related. There was one example in which a couple of kids thought it would be funny to shoot 4 other kids with pellets (yep, that one made the list!).
Also, I heard that France has suffered numerically more dead and wounded in the past year alone from 'mass shootings' than all the mass shootings combined in the United States since the start of Obama's presidency.