There might be a file in Arcanine's Halo 4 Assets Release thread, but it's quite likely not yet unfolded and even if it is, there's a good chance it's not going to be easy to work with. Most extracted models are very "rough" in quality and often aren't in one piece, but rather several pieces clipped together. With the priority usually going to armor it might be a while before any of our resident modelers find time to clean up weapon files, but it could still at least serve as a reference to work from if you feel confident free-handing it.
As Colbs said, at least 75% of the weapons and props you'll find here were carved in polystyrene foam (most commonly available in the form of foam board insulation) The rest are pieced together in wood, MDF, or built up layer by layer with cardboard (the "sliced" method). It is very, very, very, rare that anyone even attempts to build a prop through traditional pepakura simply because it's a very tricky ordeal trying to harden and reinforce the inside of them and make them sturdy enough to withstand any filling process (too easy to inadvertently crush a hollow piece) without warping and/or bloating.
Just out of curiosity, why do you not "believe in foam builds?"
