Others might hate on you for not doing anything, but I would have been scared myself. Having military training myself I can tell you that at no point during training or practice do you ever point a muzzle at anyone, even with it being unloaded. As for my reaction, I think it might be the same reaction I have when someone jumps out of somewhere and tries to scare me, I ready myself to defend myself. If a firearm is pointed at me I am sure that my reaction would be to draw my own fully loaded one and return the favor.
I know many might think that makes me act like a bad ass...But I am not, I just react in ways to defend myself and how I was taught.