Found a link to this over on thefiringline.com.  It's a WikiHow article covering how to react to gunfire, geared for civilians who are trying to not get killed, instead of being written from a military perspective, so it's pretty informative for those of us who aren't Marines.  The couple steps that mention your legally carried handgun obviously don't apply to us out here, but at least it gives you something to do instead of cowering and/or threatening the shooter with your cell phone.  If any of our combat veteran types have additional input, let's hear it!

http://www.wikihow.com/React-to-Gunfire

Edit: For spelling & grammar