SU35,
For my 6.5 WSM, I have been using WW 270 WSM brass and outside neck turned after resizing. Reason for the 270 WSM choice was that it was initially closer to the final product, and I also have a 300 WSM that did not want to have brass with the same headstamp.

For grins, I just ran a piece of 300 WSM brass into the resizing 6.5 die with no problem. It chambered into the 6.5 rifle. I would still outside neck turn afterwards though.

Doug~RR