Hate to get dragged into a pointless argument like this but sometimes I have to let my opinion know. Anyone who says modern day firearms are more effective than what we see in Aliens is out of their mind. Seeing how it's set hundreds of years in the future I think we can assume that the rounds the weapons fire are way more advanced than what we have today, even the pistol rounds. First of all the rounds are caseless, which signifies a different way of propelling a bullet from what we have today. By that time new propellants would have been developed that would propel the bullets at much higher velocities than what we have now. Secondly, no assault rifle today fires a round with a diameter as high as 10mm. Most modern day weapons fire a 5.56mm round and no 5.56mm satisfies the requirements of being incendiary, armor-piercing, and explosive tipped. In todays world it is a trade off, if you wanted an explosive bullet you would be getting a bullet that could be very easily stopped by body armor. A 10mmx24mm would be about the size of a modern day .40 cal round which is much heavier than most assault rifle bullets today. However .40 cal rounds are not going to be penetrating any sort of light armor.
Hopefully in seeing AVP:R they make a point of showing that low powered modern day weapons such as pistols and buckshot simply bounce off the aliens exoskeleton, however I doubt this since the exoskelton present in the first three movies seems to have been replaced with fleshy goo.