Lookin pretty sweet dude. I'm a big fan of the Aliens props. Classic-oldschool. You're model is really lookin spot-on to the reference. I think if anything, too spot on. It looks like your reference is a toy maybe? I'd try and get your model a little more realistic by checking references for some real guns that they likely drew inspiration from while designing that one.
Primarily, what stands out to me is the porting on the barrels. You've matched your reference by giving it those cool 'holes' but, like your reference, they're just indents. If you've got the polygons, I'd really cut those out and expose the barrel inside.
Also, it looks really thick/wide. I'm not sure if that's just true to form for Aliens (it's been a while) but it does take a little of the realism away.
Also, as Kimono said, adding some extra detail work to the back end wouldn't hurt it. Not sure I'd go so far as to move the LED, since that's a pretty noticible change from the source, but if it was going to be seen in a FPS, that thing could get a little boring back there. It was obviously designed as a movie prop, not a FPS prop.
Anyway, as far as your reference goes, it's looking really great. If you're just trying to match that, I think you're golden.