The SB2 shotgun frames are only for 10ga shotgun and 12ga slug barrels, they're too big to fit a rifle barrel to, those barrels are ~.100 larger at the barrel swell and the frames have a larger opening to accomodate them. The Superlights(SB2-SC?) and Buffalo Classic/Target(CR) frames are all SB2 rifle frames, the SBS are just scoped versions with SB2 frames.
Tim