Presently I am sizing .30-30 brass by first outside sizing in a conventional RCBS sizing die WITH THE EXPANDER/DECAPPER REMOVED. Then I push the cases up into a Lee 303Brit expander/decapper that I've mounted in an oversize (.30-40 Krag) Lee die. This does two things:
1.) cuts way down on stretching of case necks, as I'm pushing the expander into the case neck instead of pulling it through from the inside.
2.) expands the case necks to 0.307", a few thousandths larger than what the conventional .30-30 expander button does. (The Lee 303Brit plug measures 0.395". Obviously spring-back is occuring.)
Finally, I do a very gradual flare with needlenose pliers or a burnishing tool.
All of my cast boolits for this caliber are sized 0.311-0.312". So I probably have more neck tension than I need (0.004-0.005"), and with the soft lead I mostly shoot, I might be damaging the bullets in seating. And possibly also causing some eccentricity (runout) in seating, though the bullet seating operation is smooth and does not require much force.
If I added the Lyman M die to this operation as a final step, can I expect to improve bullet alignment in the neck and possibly achieve better group consistency? Presently I get excellent groups sometimes but the erratic results lead me to believe inconsistent seating/alignment might be a cause.
If an M-die is advised, which die and which size plug should I order?
Thanks. Look forward to your remarks.