Re: Necking down a case from 378 weath to 338-378weath?
I'm skeered to ask why you didn't just buy 338-378 Wby in the first place.
couple of things to look for
1 watch out for split necks. If it looks quesionalbe throw it away.
2. The radius will be different on the shoulders and you will need a mild load to blow out the shoulders correctly.
3. The double radius shoulder is designed for gas flow purposes, not strength. so when you begin to apply a lot of vertical pressure from the die you can actually push the neck right down into the case.
That's all I know and I have never attempted to do what you are doing.
The Smokin Fur Rifle Club