Udet, those were some of the greatest sci-fi movies of the 80's. "Aliens" was directed by James Cameron, of "Terminator" fame and went on to finally do "Titanic". Pvt. Hudson (Bill Paxton) always appears in Cameron's movies - he was a thug in beginning of "Terminator" and the salvager in "Titanic".
The gun in "Predator" (and I am in know way knowledgable in this) I believe was not something you could do like in the movie. What with the battery pack and other stuff to carry, I think it was made up for the movie.
And don't bother with the sequels to either of those movies. Pure garbage. "Alien", "Aliens" and "Predator" should have been left as they were.