It's cool that you're researching weapons, but I don't think you should worry about it that much. Just make stuff up that sounds reasonably plausible and it will be fine.
You see, I'd completely agree with you if I was making any other game, but Aching Dreams 3 is not a hentai game. It's a Science Fiction Hentai Game. The Science Fiction part is equally important to me as the Hentai.
I am not a firearms expert. I'm learning about how guns work though, because guns will be heavily used in The Dark Planet. Like you suggested, I could simply copy guns from existing weapons...
Recognize any of these guns? Battlestar Galactica used many guns from real life. It's a show that featured humans from a different side of the galaxy that had no contact with Earth whatsoever, but they somehow have our gun designs.
And on the flip side...

This is Masamune Shirow's work. He's a genius and one of my main inspirations. He designs these guns and people love them. She's holding a magazine fed shotgun sidearm that takes various forms of ammunition. In fact, Shirow's FICTIONAL weapon designs are so realistic that they've actually been used in Battlestar Galactica as a weapons prop. He should sue.
The M41A Pulse Rifle from Aliens.
Richard Deckard's "Blaster" From Blade Runner. The pistol is so popular that it shows up again in Battlestar Galactica and Fallout New Vegas.

Now here's the primary pistol for Aching Dreams 3. Before I went into researching, I just had a normal pistol in mind. After researching, I came up with a number of ways to enhance the pistol, make it unique, and integrate it into the storyline. It's a taser pistol now that shoots wireless taser rounds on contact with skin. With all the sensitive space station equipment on board, you'd think that the standard sidearm of security on board the station wouldn't be very destructive. Safety first! I'm not fooling any gun experts with this, but I hope they can appreciate the work I'd put in.
Now I can write lines like
Zoe- Give me a real gun, Captain. These I.S.S. Electrics aren't going to stop those zombies, and there's no piercing power to speak of.
Maybe, just maybe, the viewer will take his left hand out of his pants, rub his chin, and say "Hmm... that gun is cool."
I intend to design these guns. The same research treatment will go into the Space Station's design, as well as the more hard science fiction physics/cosmology ideas that will come into play during the game.