I've never sized a .30-30 up to anything larger. What I did was size the .375 down to .357 at the neck and load it into a .30-30 Ack Imp chamber, but the bore was .357. I'm not sure how the smith cut the transition into the .357 bore. He has since passed away.
Also I haven't yet gotten into replicating old cartridge velocities with modern pistol powders too much yet. I am still relying on advice from those who know better and have been doing it longer. So, the few loads I have were given to me. As soon as I finish remodeling my house I hope to learn more.
As I recall, the .30-30 case thins out a lot as you approach the mouth of the case. It might not be a good candidate for sizing up.
That may be why the .357 Herrett was shortened, to move back to a more substantial part of the case wall to support necking up to the .357 bullets.
[This message has been edited by sensop (edited January 12, 2000).]