Any sane person in handcuffs would be laying down in the floor board after the first shot came through the window.
Given they had their wits about them. In an honest analysis of myself where I don't compare myself to Jason Bourne, I'm not sure I would be quick or thoughtful enough to do any more than lean down in the seat, given the couple of seconds I would have to make a move. More likely I'd be right in the middle of, "What the hell is..." and then I'd get popped in the brain holder.
Those cops (as well as the victim, obviously) are really, really, really, really lucky that no shots made contact.