Wow! I'm shocked this argument is even taking place!
My 2 cents: the aliens weren't bullet proof. It just takes more rounds than we could put into them. In the movie, most of the weaponry used (pulse rifles and smart guns) used armor piercing rounds, referenced by Gorman in the hive scene. In isolation, we didn't have access to military grade weaponry or armor piercing rounds. Going back to the second movie, Vasquez had to unload a whole clip from her side arm point blank in an alien's face to kill it, and that ended with her foot getting acidified! Gorman similarly unloaded all his side arm ammo into an alien close range shortly after. This seems to support the game's scenarios where you can't kill the alien with a measly revolver before it closes the gap it disembowls you. As an onlooker, we DO see acid sprays off the alien's body as others shot the alien too, so it WAS damaged by the shots, just not enough to kill it. Then there's the shotgun... In the movie, the only alien we actually see hicks kill with a shot gun is the one he shoves the barrel into its mouth (soft spot, so to speak). My opinion is that the shot gun would be about as effective as the revolver against the alien hide but with a wider area of effect. Those marines' armor piercing rounds were pretty critical to shooting down the aliens, apparently! I suppose the bolt gun could have done more damage, but it seemed like you always ran out of ammo before an alien encounter comes up again (at least for me anyway). So I'm not disturbed by the guns at all...
... What I am disturbed by is your high praise of A:CM and low opinion of A:I?!? A:CM's had tons of plot holes, which have been extensively discussed all over the place. I don't think anyone has the energy to reiterate them here, so if you're really curious, I'd recommend heading over to the A:CM forums and thumb through the old discussions there. But I would more highly recommend that you don't. I was very disappointed with A:CM, and if you weren't, then you should preserve that by remaining ignorant to it.... Ya know, ignorance is bliss and all (I mean that in a nice way, not as an insult) :-)
PS. Looks like someone beat me to the armor piercing rounds point... Not sure how I missed that before my post. Lol