If you are in pepakura for the gun you have a few options..
You HAVE to resin the outside at least.
The best results for a cheap and fast way to fill it would be a thin coat of rondo to the entire inside, and then use expanding foam.
The expanding foam on its own will work okay, but it can bulge out the gun if you use too much. The rondo will help stiffen the pepakura enough so that the foam can be the main frame without it ruining the look. Be aware though, some expanding foam needs air to properly catalyze, and a solid gun will not be able to give that without a few exploratory holes or just using the foam a little at a time (fill from front to back or something). But the foam can be a good way to get really good solid fill for a low cost (like 6-20$). Justinian uses one that is incredibly dense and ruined one of my grinding barrels on my dremel, I usually use the cheap-o plumbing stuff, it gets good stiffness but it can be pushed in with your finger like Styrofoam (but way stronger), so the rondo layer just adds that wall to protect the foam.