Looks pretty good.
A few comments:
If I am not mistaken, you are missing the knob on the charging handle (the thingy above the trigger that selects the mode of fire).
The ejector port should be inline with the barrel, not above it.
The "gas pipe" (the smaller cylinder above the barrel) does not "join" the body in such a fashion. The top half of the body should not be a block, it should be a sort of "ridge" like protrusion, with the body under the sight resting on a block. Its hard to explain without a photo, but you can find a few on Wikipedia.
The front sight should have circle or a semi circle with the sight inside it.
Ignore my real life accuracy comments if you are not aiming for such a thing or if I got the variant wrong.
PS. If anyone is wondering why I know so much about this particular subject, me and my friends were thinking of making a FPS like Quake back in high school and I did lots of research and modelling for it, only for game to turn out to be vaporware! :p
C. P. U. Its not a big processor... Its a series of pipes!