The shapes are boulders. While the photographer probably placed the rifle on the body, I doubt he moved it from where it fell.
And yes, the free-ranging hog was not just a southern thing. In those days, it didn't make much sense to keep hogs penned up when there was plenty for them to eat in the nearby forests and you didn't have to feed them all summer. In the more remote areas, it was likely that not all were rounded up and those that weren't turned feral.
The hog is a strange animal; it doesn't stop growing until it dies -- often from its own excess weight. And no fence ever built can keep a grown hog in or out of anything it really wants; but all it really wants is food -- if it's eating well outside of your garden, it will stay out.
As Border mentioned, a hog, like a cat and few other animals, will go feral almost immediately when turned loose. Few animals are smarter than a hog.