Final update for now :
M4 with RIS
Previous Renders and Modifications:
Very nicely modelled.
The lighting looks really bad though and does not show off the nice modelling job that you did. Try using another lamps with some softer shadows along with the AO, or GI, (as I can’t tell if you rendered with yafray).
Heres and updated version. I took out the plane background and left the Blend. I also fixed the magazine, the spaces were to dark originally, now the gap is smaller and it looks better. I changed it to an original m4a1 also.
Try Yafray GI for a less grainy render. I’m guessing you used AO which is grainier unless you set it to large amounts of sampling which takes forever.
I actually like the grainy look, makes the gun look battle worn. It rendered under a minute, maybe even under or around 30 seconds.