PS. The majority of the terms used for the anatomy of a double rifle are also used with a double shotgun. There are a few differences between British and US usage.

Also, like a double rifle, a double shotgun should be regulated so both barrels shot to the same point. In the case of the shotgun they are regulated for a range of 40 yards. Since the shotgun throws a pattern the regulation is not quite so critical, nobody notices if it's off a couple of inches. Shotguns are generally regulated for a shot load and will not shoot slugs from both barrels to the same point. An exception is the Bernardelli slug gun which had the barrels regulated for slugs at 100 yards. There is also such a thing as a ball gun which is smooth bored but otherwise like a rifle and regulated for shooting gauge sized balls. Then there is also the Paradox type gun which has the last two or three inches of the barrels rifled and shoots gauge sized bullets. The Paradox will do reasonabley well with bird shot. (Paradox was a trademark, various makers used various terms for the same thing. Today we call them all Paradoxes, sort of like calling facial tissues Kleenex.)