Cerwin, best accuracy with arms and ammo only happens when everything is as repeatable as humans can make it. This includes the bolt closing and locking up in exactly the same place for every shot. If there's any binding when the bolt's closed on a round with a "snug" fit in the chamber, the bolt won't be in the same place every time this happens. When the round fires, any strain, however slight, on the barreled action effects how it wiggles when fired.
Benchresters neck only sizing would notice this after a case had been neck sized 4 or 5 times; sometimes less. So they sized such cases with dies that set the shoulder back so there's at least a couple thousandths extra room length wise for the round to chamber without the bolt binding.
Therefore, I suggest you do something to ensure your fired case shoulders are set back at least .001" when sized. Here's a link to something very handy for making very accurate changes in sizing die height:
Was that ridge on the neck at the shoulder junction intended to better center the case neck in the chamber neck?