I must have missed the projectile.
There's that scene when Lex kills an alien by using a cannon of some sort, probably using hyper compressed air. This cannon was used by Weyland's team when moving down the tunnel to plant lights into the walls.
Yet, when Lex grabs the gun to kill the alien, I don't remember seeing any projectile at the tip of the muzzle.
Maybe I'm missing something, maybe it's that crappy video, but how did it kill the alien?
Big focused wind?