The most you should size the neck down to in one sizing is about .020"-.025". To go from 308 to 6.5, you will need to size down to about 7mm, then 6.5. Plus, the shoulder bump. What's the best case to start with, 243 Win, 22-250, 308, I don't know. I just buy 6.5 CM from Hornady. Suppliers are just getting the next shipment in of them now, so keep your eyes open, and you can find some Hornady 6.5 CM. I just got in 5K of them.