personally, i think few of us really know what we'd do in this situation. all of this is armchair quarterbacking to make us look good or feel better about ourselves.
it's like someone sig that says "of course i'd legsweep him and crush him. typical atot tough guy"
I think many of us (probably older) know enough about ourselves and our abilities to have an idea.
Make the scenario better. Let's go with unarmed. Unarmed, open ground, cuckoo with cocoa puffs blazing away? Yeah, just consider anything past the next 30 seconds to be a gift from God/Allah/FSM/Science/ATOT/whatever.
Look around the plaza/mall/park. Mom with her kids there. Those damn teenagers on their skateboards. Cranky old lady. Gramps with his walker. The middle aged polo wearing guys. Basically every slice of Americana in fear and danger because of some dipshit.
In that case, helping others get out of the way is probably where I'd be at. Possibly chucking some damn rocks at the shooter if that's what it takes. Heck, you notice a lot of these mass shooters turn the guns on themselves as soon as they encounter resistance.
Win or lose, live or die, go out fighting and helping, not as one of the cowards who ran away and let everyone die.
(Note: this is not to imply that when the first shots ring out, going flat and doing a quick head/tummy/nuts check wouldn't be in order. Followed by some sort of a holy shit! moment while working out something like a piss poor plan most likely modeled after a Die Hard movie or something.)
(edit: I have a short temper. Rather than run away, I'm most likely going to get effin PISSED at whoever is ruining my day and want some vengeance. Eye of the Tiger will start playing in my head, replaced suddenly by a terrible mashup of themes and music from Backyardigans, Chuck & Friends, Thomas the Tank Engine, Shaun the Sheep, Timmy Time, Veggietales, and so forth. A red blood lust will come before my eyes and the shooter will face an unprecedented wrath at that point.)