From my personal experience of almost being an armed robbery victim:

I was with a friend in an empty train station on the wrong side of the tracks after midnight on a weekend which is a high risk situation by just about any definition. A middle aged guy of average height and build sort of waved at us and began to ask us for directions as he walked towards us. He came up to about arms length from us as he was still saying he couldn't understand the directions we were trying to give when he suddenly put his hand in a coat pocket and said "This is a stick up, give me the money or I'll shoot."

So if it happens, it'll be up close with no time to think about it.

So what happened? At that same moment our friend we were waiting for drove up into the parking lot and the BG saw the headlights, got scared and ran off. Still wonder whether he really had a gun or it was a con job.