He gets shot in the neck, it appears to exit the front of his neck and enter the backside of the guy in the front seat (I think the governor).
He then gets the front of his head blown off, which definitely looks to be an exit wound as the front of his skull flies off and mist flies forward. An entry wound would not explode out like that. The thing that throws people off is that his head moves back when the front blows off. I think that would be a natural movement from the force of the bullet exiting and blowing off the front of the skull but you also have to remember that he had a terrible back with limited mobility. As a person who also has a bad back, if you get pushed forward to the fullest of your flexibility, you will recoil back with force.
I was raised to believe in multiple shooters but I don’t see it here. One shooter.