Re: Major runout problem.
Several things cause bullet run-out, doubtful that a shiney expander can fix any of them.
When some rounds have little runout but others do it's commonly due to poor case necks and no die change can fix that. To "fix" such necks, many of us lightly turn our necks, skimming off maybe 60-80% of the neck circumference just to even things up a bit.
Another common cause of run-out is having too much "bullet tension." Or more accurately, too much difference between the bullet and neck diameter. Any difference greater than about 1 1/2 - 2 thou means absolutely nothing to actual bullet tension but it does greatly increase the required seating pressure. Increased seating pressure almost always increases runout.
The shape of an expander button can increase the rate of bent necks. A long bearing surface, such as what we have in Lee's expanders is best while a "ball" is worse. A ball has minimul neck contact which allows even slight variations in neck thickness and/or hardness to give way a bit more in that spot and that drifts or "bends" necks in that direction, at least somewhat.
Non-concentric sizer and seater dies can do it but since you are getting some good rounds mixed in with the poor ones make that seem unlikely for you.
Bottom line, it's much more likely your's is coming from the cases themselves.