Excellent geometry, but...
So, the model itself is great. This is the basic structure of a truly believable shotgun, and well over 50% of the work has been done, and it has been done well.
There are a few things that really hurt the model itself:
(1) More beveling and/or smoothing. Almost all the edges are too sharp.
(2) Reduce the reflectivity. The finer details very hard to see right now.
(3) The pump has two issues. It needs more room to move back and forth to reload, and you should make a "hole" in the pump for the shaft. We need to believe that the pump moves on the rod, and isn't attached to it.
Otherwise, this is great work and I can't wait to see more your stuff!