No The last referenced smooth bore rimfire shotgun that I can find, was the Marlin Model 25MG "Garden Gun" made 1999 - 2000.
Of coarse that's what slugs do :)
Rifled bore and Smooth bore
Yes it could be done, but you would have to locate a smooth bore barrel to change it with. Remember smooth bore barrels are not as accurate as a rifled bore barrel.
Almost any Combination BB & Pellet rifle is smooth bore. BB rifles usually are Smoothe bore. The Crosman model 760 is such a rifle and can be purchased at most sportingood stores. Look at the packaging and it will say Smooth bore. Smooth bore barrels are less accurate than rifled bore barrels. Cabelas, Basspro, Big5 and Sports Authority usually carry them.
A muzzle loader shotgun
Training.
Rifled, unless you have one marked "shot only"
Yes
Yes
Smoothbore
smooth bore
smooth bore