how do you know who is who?
It's a terrible idea to buy a gun and start dragging it around in public without training.
A carry license is NOT training.
With proper training it's not exactly rocket science to observe, plan and return fire.
Mass shootings are very different than single target shootings. There's no motive outside of murder meaning someone's going to be shot before anyone can react. The goal is to preserve as much life as possible, in the scenario we are discussing, prevention is most likely impossible.
Now that we have accepted the fact that the shooter will likely get quite a few shots off we can look at how to neutralize the situation.
Run and find cover.
Identify target.
Ready weapon.
CLEAR YOUR SHOT, the pigs in Denver just shot 6 or 7 bystanders because they fired on a suspect without making sure they had a clear shot.
Leave cover and fire on shooter
Remove weapon from shooter.
Now I want to be perfectly clear: in this process of taking cover, identifying and waiting for an opportunity to return fire, there will be additional casualties.
But unless you are standing arms length from the shooter (unlikely) your best bet is keeping distance and having a defensive advantage even if it's just a trashcan. You also don't want to be mistaken for the shooter yourself.
The shooter is most likely going to have 15-30 rounds before they need to reload, without the shooter being extremely well drilled himself and wearing some sort of chest rig it's going to take him a good 5 seconds between mags, which is a perfect opportunity to take the shot.
While it's shitty to let a mass shooter get a full magazine off into a crowd, its infinitely faster than waiting for the local pig gang to put their cosplay outfits on and quiver in their boots 2 streets down from the shooting like we have seen in not just Texas, but the majority of mass shooter responses in recent history.
Lastly, this is a reddit comment and is in no way shape or form replacement for professional training and both live and dry fire drills.
PLEASE do not carry a weapon in public especially to large gatherings without seeking proper training and spending twice as much time as you think you should on drills.
And always remember, the violence in our fucked up culture isn't because of our lack of gun laws. Gun ownership rates in other countries do not follow a trend. Sweden has approx 1/2 the per capita gun ownership of the USA, yet they have 1/1000000000000000th the amount of gun violence.
These shootings have everything to do with American culture. From letting our children become totally hopeless before they're even out of school to "going postal" because our anti union workplace culture has lead to dystopic conditions to the open brainwashing of the uneducated christofascists both in the church telling them God will always forgive them and as soon as they get home from church on FOX.
You could arm every man woman and child in western Europe and never see the sad disgusting tradegy that is American gun violence. Unlike Americans, Europeans have a reason to live, a chance at education no matter how much their parents earn, the compassion of a system that doesn't enslave them to either terrible paying jobs for healthcare or lifetimes worth of debt from a single hospital visit.
Gun violence is a symptom of the sickness that is conservative fascist capitalism and both red and blue parties serve us these disgusting policies that bring us closer and closer to civil war and violence.