Apparently, the flood are immune to sniper shots. I never realized that until yesterday. I was playing Keyes, the part where you fall out of the ship, and there where some flood on the ledges. Far out of range for my shotgun, and i didn't have the pistol equipped, so i used a sniper. To my surprise 8 shots does nothing to the flood. I tried it on a few other occasions and no luck either. this has to be a glitch right?