If you measure the outside diameter of the bullet at different points, you will find that the length of the bullet that is actually .308" in diameter is very short, especially on a boattail bullet.

For 30 caliber cartridges, I no longer use the expander ball. I use a universal decapping die, then FL RCBS die without the expander ball, then use a Lyman 'M' die to expand the caseneck and put a small flare on the neck to make seating bullets easier.

Its a few extra steps, but you end up not having to worry about lubing/delubing the inside of the case neck, stretching the case neck unevenly when the expander ball is pulled out of the case, and having the little flair on the neck prevents bullets from being scratched when seated.