Bushmaster...
Not that you are not getting good advice here... but David Tervallion and Mike McIntosh wrote a book called "Shotgun Technicana." While it can be difficult to find and sometimes a bit pricey, I think it is EXACTLY what you are looking for... explains in nearly endless detail the differences and function of almost all things double barrel shotguns as well as the history of how each improved on the previous, etc...
(the book can be found for around $50... I would not personally buy the first one that you find for $200)
