Its all about rifling. Not the shell..
Think about it.. A 22rf in a rifled S&W K-22 Masterpiece is simply not a shotgun just because you are using CCI shotshells. Same with a Taurus 85 38spl snubby using shotshells, or a 44mag Ruger Blackhawk using them. Its not the shells, its whether the bore is rifled or smooth. ..Same holds true with(except in CA) with a Taurus Judge, BFR, Contender, Comanche, Bond Arms or American Derringer using a .410bore shell in an elongated .45Colt chamber.
A 12" barreled Contender pistol using a typical rifle cartridges of 30.06, 30-30, 45-70, etc doesn't make it a "rifle".