There isn't anything other than anti climactic-ness that prevents the aliens from using guns, isn't it?

I think the answer is that the xenomorphs simply aren't intelligent enough to use guns, or simply cannot use guns. We almost never see them stand on 2 legs, and their hands don't really look like they could hold one

A human isn't born with the knowledge to operate ( or create) a helicopter or a cell phone......So.. I see no reason the xeno can't 'learn'? Probably just needs an education.

Captain Cold

I think that the Xenomorphs are technically intelligent enough that they could use guns, however, they don't need to. They're perfectly adapted to hunting prey without advanced weaponry. The only weapons they need are already built into their anatomy, with their claws, teeth, tail, speed and stealth, guns would be pointless. In a way, that makes them more dangerous than humans, because they don't need technology to kill their prey. They don't need to carry weapons because they ARE the weapon.

I actually think you are right in that regard

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A human isn't born with the knowledge to operate ( or create) a helicopter or a cell phone......So.. I see no reason the xeno can't 'learn'? Probably just needs an education.

Who would be there to teach them? David's fate is unknown even though he most likely survivies i doubt he is going to teach the xenos how to fire an AK

I think it depends the Xenomorph is Intelligent, but i think their Killer Instincts would mean that would sense their Victims and i think they would GO IN for the Attack or Sneak around rather than to STAND BACK and Shoot.

Can they be Taught? who knows it seems you cant REASON with a Xenomorph and so it could be UNLIKELY you can Control and Teach them, and BESIDES i think seeing Xenomorphs with Guns would SUCK.

Ranged Attacks by Xenomorphs should come from some kind of Mutation/Evolution and so maybe they can Mutate so their Tails and can SPRAY ACID? Or they have like Stingers  that can be SHOT and Regrown so they become like Darts that contain a Toxic then Breaks Down its Victim from inside.

A Xenomorph/Human Hybrid that will Learn to use GUNS is a Different Matter and i think this would be Plausible.

Captain Cold

Wait the xenomorph for the most part are bipedal especially in the first two movies