Most rifle dies undersize the case neck considerably when sizing. The expander ball pulls on the case when you move the press handle up and opens the neck to a compatible diameter for the bullet so neck tension will be optimum.
Several issues here... if you don't use an expander ball like ricciardelli suggested or your expander ball is undersized, you can collapse the case when seating a bullet. You can also damage the bullet because it is now being used as an expander ball and the bullet is much softer than the expander ball in the die. Neck tension will be more uniform and bullet damage is minimized if you use the standard expander ball supplied with the die. These issues can have a considerable impact on accuracy.
Brass cases have some typical properties. They are seldom precise in thickness and seldom are uniform hardness. This undesirable characteristic can affect accuracy because the bullet may not get started into the bore the same with each round. A compounding issue is when the cases get hot from being fired then are later sized. This will "work harden" the brass making it even harder to size and seat a bullet and easier to create weak spots. When you fire the round again, the weak spot may create a crack in the case neck.
The best solution is to anneal your cases from the shoulder to the mouth. This will soften the brass and allow the case to be sized and a bullet seated without causing weak spots. Your ammo will be more accurate, less soot and blow-back because of a better seal, and the cases will last considerably longer. It take about 4 times fired and resized to reach max brass hardness. Annealing every other time fired will keep the brass soft enough to minimize damage. Don't wait until the case is on it's last leg to anneal, do it after the first reload. When sizing, a little lube in the case mouth will help a lot to keep from creating weak areas plus it keeps the neck from stretching and makes the press handle push back easier.
