Klaw's got it, buy the Mossberg bbl for the 870. If you ever have to go to court civilly for wacking a bad guy with your SD shotgun, the lawyers cannot slander you for using (indefensibly) a "sawed off" shotgun.
Liberal judge jury will love that.
Much better to have a deer bbl, or a barrel dedicated for SD use.
Criminals in the eyes of judges/juries, used sawed off guns.