Papercraft guns are the new origami. The new origami for boys, that is.
WonderHowto and YouTube are chock full of paper gun tutorials. Many models are simple, but some are impressively realistic, not to mention functional. My personal favorite (unfortunately no tutorial) is the homemade Desert Eagle .50 cal by YouTube user Changmobile.
"Made with cardboard and paper maché. It has a slide, safety switch, moving hammer and a magazine release button. I spray painted it with silver paint for car rims to make it shiny."
Changmobile's Desert Eagle .50 cal: