Re: which 30 magnum most accurate?
I would pretty much agree with BountyHunter. The 300 win and wby are both exellent cases and the Norma brass would be my pick. The wby mag is usually a little more versitle due to its shoulder and neck design.
You cannot go wrong with the Ultra mag either if you want high velocities. If you are looking for velocities closer to the win and wby mags, take a good hard look at the short mags. They are beltless, more inherently accurate and way more effecient. Plus the added benefit of a shorter more ridgid lighter action. Also stick to a 26" barrel to reduce barrel whip. Great things are to be coming and have already from these short fat magnums.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.