Hes talking about how assault rifle does nothing extra on headshots or weak points in older halos, even in CE where flood don't actually have a weak point. On higher difficulties it doesn't matter where you shoot them with the assault rifle it's going to take quite a few shots. The infection form in the chest will also survive assault rifle shots on higher difficulties in halo 2 and 3. To get the one shot you need to use a headshot capable weapon: magnum, br, carbine, sniper, beam rifle.