I will go and re-read again to find out just what the hell the pred was firing, would be more stupid it if was bullets, but it probably is, cus I don't see a reason for the Pred to have a chain gun, when the plasma caster by itself is a good enough weapon.
EDIT: well in the next page or so, it says that Isabella has bullets coming her way, but I think they are just coming from her own bullets ricocheting off the "unknown metal" in the script, and her own rounds are probably going back to her, I don't now, this script is insane.
EDIT 2: The whole RAT TAT TAT TA TAT bit kinda does give away that these things are bullets coming from the Pred.