There is a GCA definition and a NFA definition. They are different. The ATF letter to Mossberg regarding the Shockwave http://www.mossberg.com/wp-content/uploads/2017/03/Shockwave-Letter-from-ATF-3-2-17.pdf
points out the Shockwave is a GCA firearm but isn't a NFA firearm. Oklahoma law defines pistol, rifle, and shotgun but doesn't define firearm. By the Oklahoma definitions, the Shockwave is both a pistol and a sawed off shotgun as are Judges, Governors, and Contenders with 410 barrels. (The latter three are not Federal law shotguns because they have rifled barrels.) Oklahoma law criminalizes possession of sawed off shotguns unless they are transferred under the NFA. And of course Oklahoma law doesn't say that a sawed off shotgun has to have a barrel which was, well, sawed off.