If you are in a bank most likely you won't be pulling any sort of firearm unless you are carrying illegally anyways since in most cases it is against the law to carry a firearm inside a financial institution and most of them have many signs out front stating that.
In a store. It is honestly highly unlikely that you would get a clear line of fire at the BG if anyone is in front of you and the panic that ensues if the BG openly brandishes the weapon would only make that worse.
In all likelihood all you would accomplish is getting yourself or someone else sent to the hospital with either the BG's or your bullet in them especially if your shot misses or passes through the BG and into whatever is behind him, namely the clerk.
Now with that no doubt flame worthy response done. I would not clear leather if anyone was in the store because I would only further escilate the situation. I would instead be calling 911 on my cell phone or trying to get the other customers to cover as safely as possible using myself as a shield for them if needed. Once that was done if the BG was still there and I had a clear shot I would call out and order him to drop his weapon and if he so much as flinched in my direction he would be getting number 1 of 6 130 Grain .357 Hydra Shocks seeking to make a few sucking chest wounds in his hide.