It depends. If immediately rushing the shooter would result in fewer casualties, maybe that's the best choice. However, if that would only result in your gettin

It depends. If immediately rushing the shooter would result in fewer casualties, maybe that's the best choice. However, if that would only result in your getting mowed down, then setting up an ambush and waiting for the shooter is arguably better. My recommendation, if you are in this situation, is to think one more time about your chance of escape. If there is truly no chance of escape, and if there are willing confederates, create a situation where the shooter is distracted and you can get the drop on him. (If possible, such that your confederate stands a chance of escape. They could distract by running and ducking into a doorway in the other direction, for example.) If there are multiple, determined, trained shooters, it's probably best to run.