Shoulder Cannon: On most Imperial combat exoskeleton designs, this is where they fit the semi-autonomous point-defense weapons. (Also, on whichever shoulder suits you, the brackets where you prop/clamp the rear end of the Really Big Guns.) Backpack Cannons are usually more common (although still rare) for offensive weapons, since they tend to need less of a good view.
Have you seen Gintama? Loads of shoulder canon in that show 😀