My F/L die and neck sizing die both size them to .309 which is where it should be.
That's about right, and normal for standard dies. If the brass neck is .014 thick, then that gives you a .281 ID on a .284 bullet. IOW, .003 neck tension on a loaded round. Which is very normal on standard (non bushing) dies.
So, something after the resize process is causing the necks to swedge down more than this (would have to be to get a .279 bullet).
What is the neck OD after fireing a round (before you resize it)?
What is the neck OD after you seat a bullet (before you fire it)?
In my mind, only 3 possibilities exist:
1) Seating die is too small and is squeezing down the necks while the bullet is seated.
2) Chamber of rifle is too small in the neck area and is squeezing down the neck while the round is chambered.
3) Some of the bullets are overly soft or have a void between core and jacket.
That is all I can think of, and they can be measured/checked.