It is worth doing another proportions check after assembling all the details. I created a plane with my concept in the background to create this accessory.
This type of concept is pretty easy to turn into a model. I've created a plane and assigned the concept as a texture. You can switch on the opacity in the layer options as I mentioned before or use X-Ray mode (menu button on the top of the viewport window). I made the rifle with a single mesh, except the belt which was made separately. I try to plan all edges before extruding. After extruding I scaled the orange area and added a few details. It's a pretty simple way to do low poly weapons (Fig.33).
The same way works for the scatter gun. I just extruded different edges for the top part (Fig.34).
Now it's time for the most interesting and important part of every humanoid character. You can build a beautiful model, but all the attractiveness is ruined if you fail with the character's face. I always give special attention to making a head. Don't be too lazy about fixing areas that look wrong and use references a lot. My base mesh didn't have any triangles, only quads, because I'll import it to ZBrush and triangles can cause topology issues in this program after subdividing. In the first phase I worked on the lowest sub-division levels and used mostly the Clay and Move tools. Too many polygons at this stage will slow your working process. Forget about details, only think about basic shapes. I made a rough sculpt because I only needed large details here for the low poly shape. You can also repotologize it in ZBrush or Topogun, or export the base to Maya and fix it there as I did with the body (Fig.35).
This is a shot of the almost-finished model. Now we are ready to unwrap it (Fig.36).