I've done some work work with the 30-30 case, from what I've seen the 30 Herrett, 30-30 Win, and the 30-30 AI, diameters at the base of the shoulder are all just different points on the same taper.
However I do agree with SD Handgunner on going straight to the 30-30 Ackley Improved, if you have at least 14" of bbl., if not leave it alone. A 10" 30 Herrett works very well with 130gr bullets. If you need help with the Herrett just ask, mine gave me some grief but it was worth the time.