Re: how to make 257 weatherby brass?
run a NEW 7 mag piece of brass through a 264 Win mag bullet seater, then full length or neck sizer. Then run the brass through the 257 bullet seater, then sizer. I have not used a Redding competition seater in the neck down process, but the bonanza is great.
It greatly helps to put a heavy chamfer on the outside of the case mouth as an aid in necking down.
It is of great help to use the White Lee Sizing wax that runs about $3.00 per tube. I have found the Lee wax much better than Imperial Sizing wax or Spray Hornady lube. The Lee wax is a great lube for turning necks and neck turning mandrel, also.
You will end up with just a slightly shorter neck than factory brass, but that will not be a problem.