Ok, besides readingme recomending reading the stickies (and I do mean all of them, just for the sake of knowledge) I have successfully built a Sniper Rifle by using pep as a base. What I did was, after I built the enitial cardstock base I resined the outside about 4-5 times in most places. Once it was stable and could stand on it's own without warping, I drilled a few holes along the bottom and sides of the rifle. Then I filled the interior of the rifle with expanding foam in sections. Expanding foam has somethign that's called "gas-out" and thats why I made a few holes in the body of the rifle, I used a unbent paperclip to help the "gas-out" process by pokin the paperclip through the drying expanding foam, I did this for about 6 hours, poking every 30 minutes. If you dont "gas-out", when the foam fully cures it will all of a sudden suck the sides of the pep inwards, and then you have a problem (it can always be fixed later with Bondo, thats what I did in some places).
Using foam will make it lightweight but sturdy. You just have to be careful with the foam. Also I like this way better than cutting the weapon in half which, like said before, may not be fixable if you do it wrong.
Hope that helps, if you need visual reference as to what I just said, click on the link to my SR WIP in my signature. And feel free to PM me any further questions if you decide to go this way^