splattering blood and brains and gore ought to give them something to think about
Have you ever seen
a bullet wound to the head? It isn't like a Romero film. They're often quite undramatic. Besides, if you're facing a mob bent on imminent violence, you're not going to be given the time to level such a shot, nor are they likely to be looking at their comrades. Their attention will be faced solely on you
, since you just produced something that makes a loud bang and a bright flash.
I can't speak for the psychological factors, but I've had occasion to encounter a mob. Once things turn violent, there is no way to diffuse or de-escalate. Retreat is pretty much the only option unless you've got truly massive firepower, and you can live with taking a great many lives.