Umm only upsetting you... you dolt. If the Aliens got on all fours maybe they could fit through there. They maybe tall but if they get on all fours they are QUITE a bit shorter, if you saw the end of the clip when the Alien was crouching it was only a couple feet tall. What youve established is that the teenage PREDS WEAPONS are not acid-proof, there is nothing saying the rest of their species does not have acid proof technology. And why didnt she untill the end of the movie? Maybe she was one tick away from goin psycho on the Preds, plot convenience I would call it, not a plot hole.