I use a RCBS .25/.284 trim die to size my cases down to .25, then the RCBS full-length die to get it to 6mm. However, today, I think I would just buy some Norma or Lapua 6.5/.284 cases, as I hear these will size down to 6mm with just the regular 6mm/.284 sizing die. I have found two powders which work well for me, the Norma MRP and Hodgdon H4831. I have not tried Alliant RE 22 or 25, or any of the Vituvuori powders, but some of these should also be good in it!!