Looks like a sword, not a rifle.
Would it even be illegal to carry a rifle around slung over your shoulder?
Don't know. I have an old samurai sword, carried it slung over my shoulder like that picture, to have it checked out by Japanese sword experts in San Francisco. Walked several blocks in Japan Town through crowds to my car, even stopping in some stores to shop. No one batted an eye, and got smiles from people. Guess a rifle would get a different reaction.