Hi John
Zero set back when resizing is best with 30 Herrett in the TC. It does take a little time, trial and error and patience but it is possible.
TC published a small manual years ago titled “Making Your Contender Perform” In there they have several paragraphs specifically on forming and resizing .30 and .357 Herrett cases, it helped me alot when I started. Please forgive me if you are already aware of this, but if this is new info I hope you find it helpful.
Forming. 30 Herrett.
Set bottom of sizer or form die 1/4” above shell holder.
Trim to 1.605” and chamfer case mouth inside and out.
Cases will intially be too long to chamber.
Turn the size die in 1/8 turn incriments (or less) until a sized case will allow the TC to close completely and freely without interference from the case.
Dry fire a perfectly sized case to be sure the hammer strikes and rests against the the firing pin.
Subsiquent cases can be sized except for trimming in one operation after correct die setting has been acheived.
Resizing.
Best accuracy will occur with zero headspacing where the case completely fills the chamber, but not a tight fit.
I was told years ago by the TC factory reps that easiest way to determine a correctly sized case is one that will not allow the hammer to hit the firing pin when the action is closed with normal force. But when the action is opened without engaging the extractor and closed again, the hammer will then contact the firing pin. It is much easier to show than to tell.
30 Herrett cases when improperly sized with show head seperation early on within 2 to 4 firings. When the sizer is properly adjusted, head seperation becomes a none issue and more cases will be lost to spit necks. One other important note; make sure you chambers are dry, no oil or grease, otherwise casehead thrust will eventually cause your frame to stretch. One last note, a form trim die helps speed up the case forming/trimming process a whole bunch.
Hope all this is helpful. Enjoy, the 30 Herrett is a fun little round.
All the best, Michael.