I doubt if there is any difference between the two designations. However, if a reamer is made to fit Winchester .284 brass, it will not fit Lapua or Norma 6.5-.284 brass. It's just like the old Remington 6 BR brass and Lapua 6 BR brass, the Remington brass is about .004" smaller in diameter than the Lapua brass. The Winchester .284 brass is several thousandths smaller in diameter than either Lapua or Norma brass.