User blog comment:MilitaryBrat/Master Chief vs Xenomorphs/@comment-4749025-20120131222226/@comment-4248317-20120131231152

Xenomorphs (at least the ones featured in the picture) are not bulletproof, at least not completely; they can be hurt by bullets, especially armor piercing rounds (which Chief has).

Not to mention there is the possibility that the acid blood might not penetrate Chief's shields; after all, they withstood a drop from orbit.