.003 under will only shave certain brands of bullets. Boat tail ammo will seat just fine, no shavings, with an extraordinary amount of grip. Flat base bullets will shave, for those you need .002.
If you read the OP, you will notice the OP knew with a reasonably high likelihood what the culprit was (expanded necks too far due to crud on the expanding plug).
He asked for any other ideas should that not work, and many were given.
No bad advice here, save one post using a crimp die to solve the problem (which wouldn't work alone, if the brass was oversized to begin with, but COULD help if he still had light neck tension AFTER resizing).
The question he posed was how to fix the 300 brass he already sized with the too-large expanding plug. (and the answer was to check the mandrel, hone if necessary, resize them, or use a collet die, or xyz).
He also mentioned he tried NEW bullets to the same effect, so your bullet-being-pushed-back-too-far argument is not really relevant.