Did this in less than half an hour.

Version 1 had no shading. Version 2 has an extended shoulder rest and some shading, not too much though.

Version 1


Version 2

Nice start, but clean up the lines and get into shading

For the Barret you ned to extend the back alot more, add some shading and its pretty decent!

I have shaded it and added some other items (coloring in the hinge for the grip, color in the background, extended back). This can be found in the second spoiler in the first post.

The forstock needs to be longer, and the muzzel needs to be larger and more square. And the carrying handle might be abit too big aswell.

The magazine + well goes behind the handle not infront. The forstock as Joe mentioned will need to be bigger to accomdate the magazine.
The stock itself doesn't have that open style grid, not many rifles I know do have those, appart from the Dragunov but that's because it's appart of the handle, so I'd get rid of that.
You might as well remove the carry handle, most don't have them, it's just some addition which Infinity Ward thought would be cool to add to it in Call of Duty. (I'm guessing thats your inspiration).

Barrett do both conventional and Bullpup firing mechanisms.

Here's the picture I got the inspiration from.

It's a good drawing, but you need to work on shading. Good otherwise