Hi,

Nice weapon you got there. I think it portrays what it is as first glace.
I think this could be improved by looking at some rifle pictures on good ol' Google.
from what I saw the part of the gun that goes to your shoulder is generally flat at the top and unless this alien species has tiny T-Rex arms the grip and and the scope should probably be moved further to the right as well.
Here are some guns on similar scale to yours from a game called Cortex Command (which I highly recommend
CC has a **** ton of guns btw)

notice how the artist crams a bunch of detail into the gun with different shades of a color.
One more thing. You should post your images at their original size. The people here can increase and decrease the scale by clicking and shift-clicking the image respectively.