It looks a little small. The stock and receiver are hard to make out but it looks like a .22 of some kind. Dunno. Strange choice. Maybe just what was handy.
According to some (supposed) witness tweets it’s sounding like he went in to the makeup department at Macy’s and was shouting a girl’s name as this thing started. Suggests maybe it’s personal.
Also on the scanner they were talking that someone claiming to be the perp had called 911 a number of times to make threats and they were talking to him. Trying to get authorization to unlock a blocked caller ID.
The only thing I can figure is it’s one of those short defensive shotguns and the picture was taken as he happened to be topping off.