Re: Next step in accuracy
I can't fault your intentions, I just want to express my views about where the priorities lie. I think that time spent at the shooting bench is worth a lot more than time spent at the loading bench. If you follow my lead, I think you'll find that you've already identified the key issues.
I use an old fashioned BR technique to guage and adjust neck tension. It dates from back when resizing dies came in one flavor, Full Length.
We can't adjust the neck sizing diameter, but we can adjust the length of the portion of the neck that gets resized. Back off the die so only a very short portion of the neck actually enters the neck sizing area of the die.
The first bullet you test-seat will probably yield to the force of twisting the bullet in the neck with just thumb and forefinger.
Adjusting the die downward a little at a time will eventually cross the threshold to where it won't. This is the minimal satisfactory neck tension, and should only be used when singly chambering rounds by hand.
Add more if you're magazine feeding. Extract and check chambered/unfired rounds to see if the bullet suffers any setback. If it does, you need more neck tension.
If you do this setup for each set of brass, for each loading, you will be able to stay right on top of work hardening, and establish/maintain a measurable and repeatable degree of neck tension. Other methods work as estimates and do not accommodate work hardening. This method uses a pass/fail method to find the right adjustment, and follows the work hardening trend.
If you want this to work best, keep each batch of brass segregated for its entire life. This minimizes differences in work hardening.
Backing off the die will probably leave the base of the sidewall fully expanded. Actually, this expansion seldom has any drawbacks, and can actually provide a benefit by ensuring a better degree of centering of the cartridge base in the chamber. Cases resized this way will eventually grow to the point where they need a shoulder bump, just as neck-only resizing requires. It should also reduce brass working in the area where case head separations are more likely to occur.
Likewise, the unresized, expanded portion of the case neck should also better center the cartridge neck in the chamber. In a SAAMI chamber, this might actually do more for case neck concentricity than neck turning.
Greg