Perhaps you've watched carboard warfare but the cardboard guns are mere cut-outs. But in this video made by the "My Name Suck" channel on YouTube, it has actual moving parts and it's not flat like the cut-outs. Unfortunately it does not shoot which would have been awesome like the woodcarved G28 we featured here before.
If you got a lot of time in your hands and would like to tinker with cardboard boxes, here is a video from Blackfish on how to make a cardboard MP5 that actually shoots paper bullets. If you have an impromptu office battle and you need your weapon, then here is where to begin.
Hat tip to The Firearm Blog for this video.