I am no stockmaker (as some folks will gladly tell you), but I know enough to know you don't start carving the outside of the stock. You start with the hard part, inletting the stock to the right fit with the gun. And that is impossible without having the gun to work with.
After you get the inletting right and the screws fit right, then you can start on the exterior work. But the inletting is the hard part and the easiest part to mess up, so you don't want to do a lot of exterior shaping and finish work then mess up the inletting and have a costly piece of firewood on your hands.