This past week I've shot 9 squizzers & 2 rats off the bird feeders, all from the same window at between 15-17yds all with the .25 FAC.
Yes most of them dropped like a stone, but 2 of the squizzers leapt & flailed around for several seconds, on checking the bodies, both head shots had huge exit wounds with most of the skull missing between ear & eye
no way on earth they were not instant kills but for some reason they went "headless chicken" Sometimes it just happens.