It's not, the outside width of the SB2 shotgun frame is the same, but the side walls of the frame are much thinner to accomodate the larger 1.2" chamber swell of the 10ga/12ga slug barrels, rifle and smaller shotgun barrels are 1.1", so rifle barrels and 12ga and smaller shotgun barrels won't fit the SB2 shotgun frame.
Tim