Some TFL members carry ARs in their vehicles, though. It's not inconceivable that a responder could grab a long gun from his or her car, and head toward the gunfire.
Then they'd be wise not to start shooting down innocent bystanders in front of me or pointing their rifle at me.
If a full on gunfight between the active shooter and a would be rescuer armed with a rifle is going on, then I'd bow out long enough to see who is who before taking further action.
And again, its almost certain that the shooting would be over with before they got there.