My question is- if I have a papercraft model, can I cast it and make a mold of it? And if so... how? I wasn't to make a Fallout gun out of papercraft, then make a solid prop. Am I asking too much? How else could I make a solid model from papercraft templates?
You can't really cast a papercraft model unless you reinforce it and prevent all RTV from entering it. It has to be liquid tight. It would probably be easier to duplicate the cutouts from the papercraft templates. Depending on how complex the model is, this may take a while. Fallout weapons are pretty popular. Have you searched the forums and looked through the Classified section on getting either a pre-made one or a kit?
you can either build the prop as papercraft and then use fiberglass and some resin to harden it up. Otherwise you use those files to make new templates for whatever other material you want to make it out of, be it wood, metal, plastic or whatever.
Do a google search on pepakura and hardening it and you'll find a lot of useful info...
If it is a one off, then as mentioned, make the paper parts from thin plastic or foam.
To mold something made from paper, the paper needs to be sealed (again as mentioned) and then you make a silicone mold, but the cost went up from next to nothing to well over 100 dollars.
Also you need to consider what kind of resin you would use to seal the paper as sometimes polyester leaches and that may cause a reaction with the silicon. Epoxy resin would actually be the best to sell the parts, then you make your mold from that.