Well I love watching shows like "The First 48" and I know that if I found a firearm in the woods, it could likely be a discarded murder weapon. Usually people don't just lose a gun in the woods, so it was more than likely left there on purpose.
That said, I would turn it in to the police... I wouldn't have any use for it. Although the $ from pawning it would be appreciated. But no, I'd turn it in.