Shotgun grip needs to be...
Independent is the only word I can think of.
I'm not trying to make this as realistic as possible, but try doing something like making the main shotgun seperate from the handle.
The best example I can give would be the Benelli M4.

I'm just suggesting a design style, but if you are trying to make the whole thing 1 unit, then I don't know how to describe it to you other than make the grip a bit more defined and not so long.
Also, just out of curiosity, where are the ejection ports for the shells to exit?