I have a 257 wby that exhibits some minor pressure signs
and occasionally the shoulder area gets a large dent in
it upon firing.

The pressure signs are relatively minor (slightly flat
primers) however the denting and gas flow back around
the shoulder is troubling.

Any ideas on what is causing this??

Maybe try neck sizing only if it's possible the FL are sizing too small. You should have a nice tight chamber fit every time.



Maybe the dent is a seperate issue. Too much lube maybe? Just and idea.

Perhaps its cleaning solution or oil, which has accumulated in the neck area. A couple or years or so ago, I got the brilliant idea of cleaning a new barrel after every shot at the gun range. I ended up with too much cleaning solution in the barrel, which prevented the case from grabbing the barrel, and a action that lock up so bad, I was worried I would break the bolt before I got that action opened. The case looked exactly as you described.
This was one of my screw-ups which I write about hoping it prevents someone else from making that mistake. Make sure you run a dry patch after cleaning.