6 shots can be fired in an incredibly quick time, and I think with adrenalin pumping and having a painful injury to the skull with possibly one eye closing up, plus being in a fight or flight situation, the officer probably just did what all defensive shooters are at one time told to do – full magazine center mass. The officer probably doesn’t even recall how many he fired, - he just did.
And I’ll leave the conclusion to the CSI experts, but I’m only seeing two ways the top of the head shot could be made. Brown laying on the ground when they final shot was made, or brown standing in a chin down position, like he may be in if he was charging, or reacting to the first hits. No witnesses, said he was lying down when shot.