Home invaders can yell "police". Some police don't wear uniforms for no-knocks.
Confusion abounds.
What agency serves no-knock warrants with no uniforms? I have seen lots served... with uniforms and a couple plain clothes mixed in, but even they (plain clothes) wear the "raid vests" with POLICE all over it and a badge around their neck...
Shooting in this situation will most likely end your future as a free citizen.