It’s somewhat ironic that the NRA’s press conference was disrupted since the NRA has a history of trying to disrupt research into gun violence (Bloomberg - Are you a robot?). Also, it was a press conference at which they would take no questions from the press.
And of course, there was an armed police officer at Columbine, but yeah, we missed the point.
The thing that I find is overlooked in discussions like this - the gunman has a terrible advantage, in that everyone else around him is a target. Also, the gunman tends to be indifferent to his own safety, often killing himself rather than be taken in alive.
In contrast, whether we are speaking of police, SWAT teams, military personnel or armed civilians, they are at a huge disadvantage, in that they have only one target and a lot of very tragic mistakes that they want to avoid making.