An internet forum is probably not the place to get this kind of advice, so I have a few suggestions for you.
Please read the CT Statutes carefully regarding the use of force and deadly force (531-18 through 53a-22): http://www.cga.ct.gov/2011/pub/chap9...#Sec53a-18.htm
If you have questions about the statutes consult a competent criminal defense attorney for clarification, several names have been brought up here, Ralph Sherman in NB, Rachel Baird, Martha Dean all come to mind...
Finally, find some advanced tranining courses here in the state that in addition to shooting under stress, low light, tactical and whatever also cover the self defense laws here in the state and discuss them in detail, usually a lawyer is brought in for that purpose.
ETA: Since you asked what would YOU do the best answer I can give is there are to many variables to your equation to give a firm answer, you have to consider a lot of things, like how many others are in the BK, where are they, does this guy have "friends" inside or outside, I think the first thing I would do if I could would be to use my phone to call 911 or local PD, believe it or not I have every CT local PD number in my contacts because 911 from a cell goes to a centralized 911 call center, even if I can't talk I want the line open to record what is happening. If the "gunman" had his back to me and I felt he was operating alone I would probably bash him over the head as hard as I could with something if I believed he would be unable to injure anyone once I hit him, if he has a gun trained on someone that may not be the best approach as he may fire the gun when struck, another alternative would be distract and disarm, not recommended if you have never had some training in that area and again you have to keep a watch out for bystanders who may get injured. Introduction of my firearm into the fray would be the last option. That's the best "hypothetical" I can give you for what I would do.