I would use 'at' here, when one person is attempting to hit one other person. For me, one person with a machine gun (i.e. with a lot of bullets), or several people with any kind of guns, can fire on a group of people.
If you fire at somebody, you intend to hit that somebody. If you fire on a group, you don't necessarily intend to hit any specific person.
That's just a personal view.
Student or Learner