It's looking really good, so far, but if you want my feedback the crosshair creates a visual imbalance. It's much too flimsy compared to the rest of the weapon, which is solid and heavy. It also breaks the otherwise very clean lines. Maybe you should consider that - so if you want iron sights, maybe you should consider making the iron sights much more solid and in tune with the rest of the design.
The gravity loaded ammo box looks better than the ammo box, but I still believe it makes the weapon very lopsided and breaks the otherwise very beautiful gun. You could consider trying to work with another design next to this, for example where you include a sliding bolt mechanism in the side of the shotgun - this would also serve to break the monotony of the sides.
Finally, a few minor suggestions to consider - reshaping the stock slightly, to accomodate for a handle with a butt stock extension (take a look at some real shotguns, you'll see) - I think it would work well with the already very fluid design of the barrel. Also consider extending the front of the trigger guard in a concave shape, maybe? That would further bring attention to it.
Oh, and also, consider placing the trigger differently, it appears that it will not pull from its current position.
Hope you find any of it useful, and good luck!