I have never been a bank robber, but I would expect apon his dramatic entrance, the first thing he would be looking for is anyone reaching for anything that could be a gun (I would reccomend not even reaching to hit the restart button on your pacemaker)
If he saw a gun come out, that would spell disaster for the person with it. You might be the one and only casualty, and the EXAMPLE he points out what will happen if someone else wants to "play". I would reccomend you do nothing, and be home that night to cuddle your loved ones, rather than the fool trying to be a hero to save a bank a few grand. Just MHO.... I'd still be there trembling..... so ignore me if you like
(Added later)
Also, If a person (you) was seen by another ccw, holding a gun, he might think YOU are the robber and do his civic duty to dispatch you? or the cops might take you out thinking you are a co-offender. MHO, unless you or the other victims are in imminent danger.... let the crook be the only one at the scene to bring a gun, hopefully then, only he will pay the price.
Hero's have their heart in the right place..... dead hero's might find their heart all over the wall behind them...
Muzza