First off welcome to the forums! I have looked at your model and I have to say good start. You have the right idea, but you need alittle help on the mesh before you start texturing. When you look at your model you will see some black faces trying to show through the mesh and some black faces in plain view. This is not good. What will happen is this when the gun is placed in fpsc it will appear as part of the mesh is missing and we don't want that. So, to fix that you can start all over or you can select each face that is backwards and flip it.
For you I would say start over, but this time you will know more on how to make it. when you make a face and it is black, press ctrl+shift+F and it will flip it to normal. With the faces in the " normal " it will make texturing a whole lot easier.
I have started a new model with some decent reference pics for you to show you somewhat how to start. Also note each side is different you need a opening for the used shell to exit. When you get to that side simpily copy the first side and flip it to the other side and then modify the mesh as needed.
Now you also had a problem with extruding the mesh from one side to the other. The problem being all those extra faces on the inside where no one will see them. To fix that problem goto vertex-extrude edges and you should get something like this:
I hope this helps you out. If you need more help feel free to ask.
