H&R bores are .0005" larger at the muzzle than the breech end, this is due to them turning the barrels after they are bored, turning it down even more will open it up a bit more, just how much I don't know, but I wouldn't turn the muzzle down any, just thin barrel so it has a fairly straight contour like the Superlights, instead of heavily tapered, which start out as the same blanks as all barrels at 36".
Tim