If no-knock warrants are legal then it shouldn't be a crime for people inside to shoot at the officers coming through the door. As far as I'm aware, in most jurisdictions, it's legal to defend your home if an armed intruder breaks in. Until the officers have clearly identified themselves, it's entirely reasonable for the people inside to consider anyone coming through as armed intruders. And I don't think in the mere seconds after someone busts down the door it's possible for the people inside to fully grasp that it's police coming in rather than someone else meaning to do them harm.
I'm not advocating people shooting the police. But if someone (even if it's a police officer with a warrant to do so) busted down the door of a police precinct and rushed in armed to the teeth, he would certainly face immediate return fire from the officers inside. And none of those officers inside would (or should) face charges.