I don't know what Bourbonnais said on the Knapp show, but in an interview here it doesn't sound like he was actually in a position to see Murray die. "He had run diagonally so when I came out he was to my left in the hallway and I couldn't see him.... She had no cover. He fired - I heard him fire three, I heard her fire three, ... and she just walked forward ... firing the whole time, and so I paralleled her not to get in her way and went to the wall and worked around to the corner, and when we both entered where he was he had slumped backwards, slid down on the floor and expired."
The coroner's office has released a statement. "The death of Matthew Murray has been ruled a suicide," the El Paso County Coroner's Office said in a statement.
"It should be noted that he was struck multiple times by the security officer, which put him down. He then fired a single round killing himself," the statement said.
You don't need to see a shotgun fired to know it's different than a handgun.
I do note that the CNN story you linked doesn't really have the corner's statement, just a quote from it. No shotgun mentioned, and in fact that story is still saying "assault rifle".