This is just the body as it's not finished, but I'm looking for advice on how to fix the geometry bugs in the back section. Also, any input on the realism of it and how I could make it better would be appreciated.

(What's not there: Pump, trigger, rail notches, fine-tuning details, textures?)

Random details~
Software: 3DS Max
Guide: http://i48.tinypic.com/2itk5zo.png / http://i45.tinypic.com/2agox9c.png (made by yours truly at school (MS Paint))

The main focus was trying to keep a relatively low poly count without loss of quality.
I used bridging, extrusion, vertex removal here and there, quickslice and cutting, chamfer, and pro boolean. If any of these are a no-no or if there's a better alternative, please point it out.

Whats the polycount?

(Without a reference number its hard to give some C&C)

Other than that;
1: How the heck do you reload it?
2: Are you texturing it, or are you just trying to make the texturer cry? (have you considered the mapping problems, and why not make it into a multi-object model?)
3: Do you have a reference?
4: Why the heck did you waste Polys to connect the barrel with the support that way?

If I recall, polys were somewhere around 700. Chances are, I'm going to entirely overhaul it. And yes, I plan to texture it. I'll probably be lazy and have the shell insertion part be part of the textures, on the side. The closest thing to a reference was a Benelli M4 while I was at school making my guide. And really, I'm not sure how come I made the barrel support like that. It's just what I decided, so I went with it.

The trigger guard needs to be smaller, and the pistol grip needs to be bigger overall.