If there is one person with a gun and several victims, it is difficult for the perp to watch them all, at least in the beginning, before he groups them all together. This could be a good time to strike, if there is an oppportunity.
People tend to think 'trained terrorists' when the perp might be a strung-out junkie. How clearly is that person thinking in terms of tactics? Maybe not so well.
Weapons can be anything at hand. When I lived in SoCal, a man with a gun tried to rob a small store, and a group of people pelted him with canned goods. Of course, this was long ago, before current times when the other customers would probably join the perp and split the take.
Oh-oh, sounding cynical again, huh?
Sue