6mmBR build or buy

I intend to either buy a Savage 12 BR or build a 6mm BR rifle. I have read a lot about the 6mm in the forums and 6mmBR/Accushooter. I'm still hazy on the differences between Norma brass and Lapua brass, and dies to accommodate the differences. I intend to use the rifle mostly for steel long range target, but haven't ruled out bench shooting. When searching for dies for precision reloading, do I match the dies to the rifle, or the rifle to the dies? Do I request a no-turn chamber and match dies after fire forming brass? I've had problems with not being able to chamber fire formed brass in my 223 and 308. I don't understand why I would want custom dies for brass that may not fit my chamber. I'm trying to understand this process before I talk to a gunsmith or die supplier. I want to make sure I'm asking the right questions. Once I've established the chamber and die sizes, will I be limited to using either Norma or Lapua brass exclusively? Thanks.