I read the Irvine World News every week it comes out. This kind of thing happens a couple times a month. Most of the time the BG acts as if he has a gun and intend to shoot people, but no gun is ever produced. He produces a slip of paper saying he has a gun and wants money. The money is forked over and this guy runs like hell.
The second type (happened a couple times in the last 6mo) a gun with a gun enters the establishment, randomly points the gun at people and items and demands money. Every dollar is handed over, and guy takes the money and goes for a getaway vehicle or just walks away.
The third type, The guy takes the gun out for all the see, totally confident, sticks the gun in someone's face, and demands money (verbally) from that person. Money is handed over, and guy takes his gun and leaves.
For the first type, its doubtful that guy even has a gun. Or he has a painted airsoft, or a BB gun. I'd feel confident to take out my gun and prevent a robbery. He'll probably run away as quickly as he can. If not, I'll hold him there (on the floor) until the cops arrive.
The second type, its very unpredictable. Maybe hes just nuts. Or he has a fake gun and tries to act as if its real. Calling the cops right away is a good choice and stay out of it. I'd rather not approach a nut with a loaded gun thats ready to kill. But once the police intervenes, I'm ready to assist them if they need me.
The third type, fake gun or not, I feel I need to intervene and try to stop the robbery. He would probably point the gun at me or run away. If he points the gun in my direction, I can legally start shooting. If I have a small caliber (like a P22), I'd try to chop his gun-welding arm off with bullets. If my opinion that more effective than shooting in the chest. If its a larger caliber (like a 9mm) then its a head shot.
The problem with this situation is that I need to be in a spot that he can see me and the gas station attendant so he is more likely to turn his attention to me than use the attendant as a hostage or something. So when I yell out that I have a gun and want him to surrender his, he is more likely to point his gun away from the attendant. And as he swings his gun towards me I could either shoot or yell again for him to drop his weapon. And the attendant would not be in the line of my fire.