how about a double that is a fully rifled 12 gage? Does anyone make such a beast?
Yes, these have been made. May still be in Europe.
I have a pair of Charles Lancaster oval bored, full length rifled barrels. Not the gun, just the barrels.
It is a routine, if expensive, procedure to rebarrel quality double shotguns by "sleeving", fitting new shotgun barrels to the monoblock. Rifled barrel blanks are available, and could be installed. Regulating would be an adventure, of course.
This would be an option, as far as making a rifled double gun from a shotgun is concerned. Discussion has been limited to use of rifle calibres, but there is no reason that a rifled shotgun could not be set up to use one of the excellent modern slug loads.
Incidentally, in the excellent thread in SuperCub's link, mention is made of balance in double rifles. Rifle barrels tend to be very slender at the muzzles, keping the balance point well to the rear. If liners are installed in shotgun barrels, and if they are full bore diameter, the gun is going to be a club. If liners were reduced diameter, and the shotgun barrel acted more as a casing, with the liner not being in contact for most of the length, balance problems could be reduced.