And still no one is offering a better, more accurate term for the event than "mass shooting". The fact that there were multiple shooters doesn't change the description at all. The fact that they had whatever form of disagreement before the shooting started doesn't change the event at all. Even calling this a "gang war" wouldn't change the description of this event at all.
You and I can agree on all of that - but then, you and I aren't intentionally using the emotional charge of the term "mass shooting" to push a civilian disarmament political agenda, nor are you and I trying to manipulate coverage/newsworthiness of incidents to push other political agendas.