BDM142 Digital Media - Issue with 3d Element - Gun

Following the theme I chose, which is Military themed poster from WW1/2 (more writeup about that to come) I chose to replicate some guns in maya as my 3d element. I broke up the gun into a few pieces, mainly the complicated and the, atleast what i thought was, the simple. I've run into snags where trying to get an overhang etc to look right just isn't working. So even more so i'm going to have to start again and break it into every visible part so the polygons for each piece is as simple as possible.