Olin Corporation (which trades under the Winchester Ammunition brand) has just been granted a patent (#8161886) for a 12 gauge shotshell design that would be used in conjunction with a barrel insert in order to allow the firing of small-caliber bullets through a rifled barrel from a 12 gauge shotgun.
Why would you want to use such a system? For the same reason you would prefer a rifle over a shotgun: ballistics and accuracy. Only this system could be used during shotgun hunting seasons! Sneaky, although I as soon as it was bought to market it would be banned by the state hunting authorities.