As far as our friend cedartop's idea of making sure you have your bases covered;
It may be totally legal to "stop" the thugs from robbing the place, but aren't individuals subject to civil suits along with a jury's judging of a crime?
What if something goes bad on anyone's part? One of those gunfight rounds that missed stops inside an innocent bystander? Legally or not, is it not subject to interpretation that if innocent's are harmed, someone will be seeking accountability?
One of your kids, siblings, etc. is flipping burgers and catches a round in the head from a "gunfight" that arguably didn't need to happen? Because mr officer wannabee thought he should save the day, your loved one is harmed or dead. Shouldn't they be held accountable?
I didn't say it but needs repeating, "every bullet that leaves your gun has a lawyer attached to it"