I don't remember the term "hard cast" when I started shooting cast bullets about 35 years ago. Seems to be a recent buzzword mostly used by newer users of cast bullets. I don't believe there is a standard designation of "hard" (is that 16 BHN? or 18 BHN? or mebbe 20 BHN?) or "soft (8 BHN? or 10 BHN, or 12 BHN?). From my reading Keith did not use "hard cast" as many of his alloys were designated by lead to tin ratio (20-1, 16-1). The term is jst used to designate cast bullets from swaged bullets that has gotten to be "the" term for cast bullets.

From the OPs first post, I'd say he was shooting undersized bullets to get the reported 6" @ 25 accuracy. Yes S&W does know more about making barrels than most reloaders, but knowing exactly what your gun measures is essential ( sizing to cylinder throat is 99% for clean shooting, not alignment or "accuracy").