A rifle model work in progress, I am working on for a story concept.
Because it’s at an early stage it’s not really possible to say much about it. The good thing is you’re getting used to working with the basics. You obviously set up an image in the viewport and traced the outline. Then extruded that laterally. Which is exactly how I did my first few models.
The best approach you can have right now is to ask yourself if you think you’re happy with it. What about it looks like it could be improved? Was tracing an outline the best method? It was quick. But did you get the result you wanted, even at this early stage?
Let’s take the barrel as a simple example. It would make more sense to begin with a cylinder primitive. Why? Because the actual real-life version is exactly that. When you’re lined up in the viewport and edit the cylinder it won’t just look right from one viewing angle. Because the radius is the same in any direction it’s going to look right from every viewing angle. Even if you only work on a side view.
Because of the nature of a cylinder I knew it would look much closer to the actual barrel. Despite the fact I only used one view as a reference.
Obviously 3 or 4 orthographic type views would be better to get a more accurate recreation. But that’s hard to do in some cases. Which is why collecting a lot of images about the subject is really useful.
So try breaking each element down to a very basic shape. See whether it’s possible to start with a simple primitive. Be it a sphere, a cube or a cylinder. Once you can establish that, it’s a matter of refining the shape by scaling and moving the geometry around and refining it with extra loops. But only what you need. If a loop of faces/verts/edges is not changing the shape in some way, then it’s not needed.
Pay attention to how the object itself is constructed. That can give you clues about where to make the mesh contiguous, or separate it into parts. Doing so can make it easier to manage. Look for clear breaks. And areas overlapping. Something like the image below. I’d take each coloured area as a separate piece of the mesh.
Keep at it. And good luck with your project. 
