I managed to avoid the Boomer artillery fire and speak with their leader. They promised not to fire on me any more, and I already performed a quest for them and have a reputation of "accepted" with the Boomers.
They still fire on me if I come near the Nellis gate or the area in front of it. They don't mind killing their own people in the process.
I don't want to reload and redo the quest for them again, is there any way of circumventing that bug, e.g. via some console commands?