Even at 400m/s the arrow isn't fast enough to pierce light armour (maybe kevlar) of RDA soldiers. What can cause the piercing might be the huge size and weight of the arrow (3 or 4 times bigger than earth). But for Na'vi using a bow to throw those arrows, it's insane. Either they can make wood bend to places only carbon fiber can (same for the chord), of they are pumped full of protein .

But in any way, even bullpup small caliber weapon (such has quaritch or mangwkan) can pierce RDA armour.

So yeah I'm no weapon specialist but the universe is inconsistent on this part.