I built a sniper rifle. It is made from cardboard, scrap wood, PVC pipe couplers and a telescoping pool skimmer pole - available for about $12 online (I had one left over from a blow-up pool we no longer had). It is 48" long and has a module to add a rifle shot sound. It took about six hours to build.
Step 1: Body and barrels
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.