I think the handle and stock are separate, you just sorta jam the handle on the bottom and the stock on the back. That's what I'd do.
Also, I sorta prefer the design of the body in the concept art to the changes that it looks like you made. I'm not sure if you want to do anything about that, but I guess I just can't help but compare the model to the concept art. (I assume you'd make it spot on in the high LOD version) Anyway, it looks good; a lot better now that I understand how it'll look when it's done.