I think we can debate where to shoot all day long,but in reality we can only practice to shoot where we would like to and when the stuff hits the fan,we will be lucky to do the job as quickly and effectively as we possibly can.I'll be aiming at the biggest target of opportunity I hope,and if at all possible would take out the brain and or central nervous system asap,but under mega stress would aim for the center mass.I would tend to think that the head bobbing and weaving might be tough to hit,but hopefully,I'll never have to find out.Shoot what you can hit.,as many times as possible until the threat does not exist any longer,then reload,from whatever you can hide behind.The number one rule is keep moving,take cover and take out the threat as soon as you can.