>In old paintings of, e.g., duck hunting scenes, you will often see the
>hunters depicted with their empty guns (single-barrel, side-by-side,
>or over-under) open like this and draped over one arm. This was a
>safety technique, sadly invalidated by the development of the
>repeating shotgun. You can't accidentally shoot yourself or your
>hunting buddies with an empty shotgun that's open and hanging over
>your arm. (Similarly, competitive shotgunners may only close their
>guns when they are immediately ready to call for a target, similarly
>to minimize the potential for an accidental discharge.)

Another reason people carried breakopen shotguns open was due to legal requirements. Back in the days my father hunted, it was flat-out illegal to carry a loaded shotgun across a road, waterway or fence in Ontario, (I don't know the current laws, he hasn't hunted in ages). By having the shotgun open it was technically not loaded yet but you could be ready to fire very quickly.