Assuming I am not with loved ones....The very first thing I would do, with the utmost priority, is to seek "cover"...not "concealment". The next thing I would do is unholster my weapon. At that point I would attempt to get my mindset out of panic mode. I would not just start blasting away at anyone else that was shooting! They could, very easily, be cops or "good guys", or gang members just trying to kill each other. If I was under "cover" holding my pistol, I would not attempt to turn my back to the shooter, leave my cover, and try and find an exit! That being said, if the "bad guy " has a firearm aimed at me from an angle that negates my "cover" I would shoot his ass until the threat was neutralized. Showing the shooter your ass and elbows running for the exit could be very stupid. Shooting at "whoever" else is shooting could be very stupid. I will reiterate, If bullets are flying the very first thing to do is find "cover".