I may go against the grain here,
but I'm going to say: stay calm, watch the situation like a hawk, get into a ready position WITHOUT letting your body language tip the robber that you are a potential threat, and hope for the best. After scanning the area to make sure that he has no accomplices, I'd watch that tigger finger real closely. If he makes any movement that would indicate he intends to fire, or if he is clearly acting negilgently with the gun as to cause a accidental discharge, then I might act. But, if I think that the situation might end with the guy getting the $ and bolting without harming the clerk, I'm not about to start a shoot out that could get me or the clerk killed.
That plus the fact that, even if I do get the jump on the guy, I still have a lot of problems. I just wanted a gallon of milk, now I'm in a police interview room being asked all sorts of questions, the DA is investigating the situation to decide whether or not to file charges and my gun, which I worked a lot of hours to buy, is now sitting in an evidence bag for an indefinite amout of time.
I'm not a police officer and it's not my job to stop crime. I carry a gun to protect myself. Now, if I think that clerk is going to get hurt, then I'll act to save another human being. But if I think that that the only loss is going to be the money in the register, then that's 7-11's problem.
No heroics here.