My 6mmBR reamer has a 0.272" neck and I have never had to turn brass, Rem or Lapua.
My Forster FL sizer die neck was honed out by Forster on their Sunnen to 0.265".
I use a Forster 6mmBR benchrest die.
I use a Wilson case gauge.
I have built 6mmBR on two R700 actions, one Sav99 action, and one...