How to do accurate hit detection with physics blend weight on?

I have a zombie like character whose upper body is simulating physics, with blend weight shifting randomly. result in a nice shambling walking effect.

But as long as blend weight is turned on (not 0.0), I can’t seen to get any head shots, the acutal collusion box is always at where blend weight is max(1.0), I tried all the mesh and skeletal settings with no success, I wonder if there is a workaround for this, please someone help me.