How can I change bone's rotation by script during an animation?

I'm using Mecanim Animator, and I wrote this code:

 function lateUpdate(){
     if (aiming){
         var cameraTransform = Camera.main.transform;
         leftArm.rotation = cameraTransform.rotation;
         rightArm.rotation = cameraTransform.rotation;
         }
     }

But the arms keep following the animation, I tryed to change .rotation with .localRotation.eulerAngles using a vector3 but nothing changes, how can I fix it?