I used a scale drawing and other reference pictures I found on the internet to get the general shape and size of the weapon. The 48" length was determined to be the best size for my Son's height. I drew a grid on the side view of the diagram and used it to transfer the silhouette to the cardboard. I made two sides and glued the inner and middle tubes and scraps of a wooden handle to add support to the structure.
The telescoping handle had three sections. The inner was used for the lower barrel. The inner section extends out to be the 'appropriate' length and the remaining length simply adds support to the gun within the cardboard.
Side detail was added with another layer of cardboard as shown in the fourth picture. Strips of cardboard were then glued between the two sides to finish the gun's structure.
The last picture shows some extra detail and how it looks with primer on it.