Yeah this is the one aspect in the thread that I can agree with, a lot of the time no matter what weapon you're holding, or how many times you shoot them, they
still stand there and then charge fearlessly.
But, they are still capable of recognising you have a gun every now and then. It seems weird in this instance most of the time they don't respond, as the reactions are in the game. Maybe a bug?
From the vid I posted on the last page:
http://abload.de/img/untitled1n8spq.gif
From one of my earlier playthoughs:
http://abload.de/img/untitled2cgs44.gif