This has turned into a labour of love and took FAR longer to complete than I would have thought possible at the outset.
Construction is 90% card with a few plastic bits and some bits and bobs of detailing. The main structure is a series of boxes glued together, a load of car body filler and finished with strips of card and a cool paint job.
The card I have used is high density 'show card', and high density card will do, the main structure is 1.5mm thick card (about 1/16") with the surface panels made of cereal packets.
Step 1: Barrel housing
The rifle is made in sections. I know little or nothing about real guns so I made this design as a hybrid of a few actual prop guns (Aliens and Starship Troopers) plus some game prototype designs I found on the net.
I wanted the final gun to be about 1 metre or 3 feet long. I used that as the basis of the gun and just made up the design as I went along, sketching each section as I went.
The main barrel housing is simply a rectangular box. I cut the top, bottom, two sides and two ends allowing for the thickness of the card in my calculations. Since the gun needs to be reasonably robust I added two internal spacer pieces.