its really hard to say what someone would do in any situation, until they have been there. a handgun vs and automatic weapon is a tough situation for even the best of shooters. if i were to insinuate myself into that situation, anything within 15 yards is a reasonable range for shooting accurately with a handgun. however you will have to be aware of what is going on, and on the scene intelligence is going to be valuable. where is the shooter in relationship to the entrance? can he see the front door move if you were to enter? where are the victims in relationship to the door? are these people going to call out to you if you go in? are you going to be able to tell them to keep quiet without being too loud? how frightened are those still in the store? are they going to possibly mistake you for a second shooter?
and the big question, can you pull the trigger and take another persons life? can you do it silently? or are you going to fall back on the concept of fair play and offer some sort of warning? are you an accurate enough shooter to hit a small moving target on the first shot?