I just recently got a custom 257 Wby and have had a bit of an issue fire forming 7mm Rem Mag brass to 257 Wby. Although there are a number of threads floating around where this is discussed, I haven't seen any that include load data. No problem with chambering or firing but I can't seem to get the rounded Wby shoulder to form.
What I've tried so far is two different loads: 1.) 100gr Rem SP with 65gr RL19 and 2.) 120gr Rem Core lokt with 67gr RL19, both loads were developed using virgin Peterson brass and fresh Remington mag primers. The reason for RL19 is nothing more than "I got some" and "I don't use it much anymore." I found load data for RL19 from Nosler online. Min/Max loads for 100gr bullets with RL19 is 65/69gr, and for the 120gr min/max is 61/65gr.
On the left is a piece of unmodified 7mm Rem Mag brass, on the right is one that has been run through my full-length sizer, loaded with 67gr RL19 and a 120gr Rem Core lokt.
Brass looked much the same with the lighter load (65gr RL19 and 100gr bullet), which is why I increased bullet weight and powder charge, which is actually 2gr over max book load from Nosler. One more detail, I checked velocity on the 120gr load and I'm at 3200fps out of a 26" barrel.
Should I be using faster powder to fire form? Could freebore be the issue here?