OK thoughts but this part is wrong. I would call 911 from a store phone, not tell them who I am, not tell them I am armed. Otherwise, I always carry a vid cam, I'd be a good witness, but would try to remain anonymous. First thought would be to flee and observe from a safe distance, but ONLY if I was alone. I would flee and not look back if I had a loved one with me. Having the HG means I have the ability to flee, basically. (who's gonna stop me?).
Let the dispatcher know if you are armed and have your weapon out. Let them know your discription and location. Keep 911 on the phone, if you can, as you change positions if you are going to go mobile.