First, this looks nice. I love the gostek and the modification of Lacey-v2, though the preview image makes the ammo count look like it's a few pixels too low.
Second, the criticism. I'm pretty picky about this stuff, and I've got a few things I think would make it better. I don't really like the SG-552's outline too much. The stock really shouldn't have a "straight" line on the bottom, in my opinion. Make it look rounder by taking out that lower-left corner pixel.
The lower-left corner pixel of the grip makes it look too pointy.
The magazine looks straight, maybe slanted downwards and forwards a little. From what I've seen, magazines usually curve forward.
Then again, I'm basing my criticism off of what PapaSurf did in the
Neo-Arsenal series of weapon interface images, which I consider to be some of the best ever created. So, if you make those changes and find they don't look as good as you'd like, feel free to keep it like it is.
Your shading is nice, if a little bit dark. Though, it is quite nuanced and I see more detail as I look closer.