Here are a few updates, I had a little time this morning and decided to fix the barrel support area with the holes. I did not pay attention to the references (and there are also a few variations on that part) and the version I liked has three holes on the side and four holes above and below and the pattern is repeated on the other side for a total of 22 holes.

That portion of the model is still NURBs as it is easiest to construct that way. The other components are polys and some of those bits I built first using NURBs. The back portion that attaches to the receiver, for instance, I built as NURBs patches, then projected the holes area the is cut out. Then converted the patches back to polys and simple welded the verts and added some control edges.