I think the wepons looked the way they did just due to the artist's interpretation. Nelson in that era was fairly loose in drawing industrial design objects. He's a great artist, but he didn't seem to have that product design sensibility. That was a bit of a shame too because it did take away a bit from the book. The same could be said about his drawing of vehicles and miscellaneous gadgets. They just look like they don't have much product design thought put into them. I mean take a look at the dropship they're using. It has the strangest arrangement of tail control surfaces that are not symmetrical. All of this was rectified by Aliens Book 2 when Den Beauvais took over. He had a bulletproof design sensibility. He is a natural simply. And then everything went to shit completely in Alies Earth War with a brilliant artist who was completely character driven with no product sensibility whatsoever.
But I will say this, Mark A. Nelson's spaceship design "The Benedict" was probable one of the best if not the best ship design since the Sulaco. I mean imagine that thing done in full CGI. Awesome!