Gun model in progress, based off of the Scar-H
Need to get around to texturing it.
Post a wireframe version and I can help you out. You are missing a lot of technique. I would suggest finding a gun tutorial (cg cookie maybe) and learn from that. You are missing even basic shapes such as your magazine clip which is rounded but all magazine clips are square.
You are also missing edge loops to make sharp edges, which causes your lighting to be wrong.
Definitely purchase a monthly CG Cookie and watch their M4 model tutorial fully. Or find another weapon modelling tutorial that is good. This is very beginner I can tell.
My SCAR model for reference. Things like the front grips or safety can float and be separate geometry (In real life they are actually separate). Your scope is way too simple. If anything I would go find a scope and stick with modelling that and posting along the way.