bjlooper,
What is the diameter of the unfired brass? Yes, the chamber may be within SAAMI spec chamber or even a tighter custom chamber, but the critical dimension is the difference between the cut chamber and the unfired brass you are actually using. Not what a book says or nothing, but actual real measurements on the unfored brass.
Brass manufacturing isn't always done the same and this dimension can vary. Sometimes you might purchase Rem brass that measures .5115" but Winchester measures .5090", and Weatherby brass might measure .510. A year later this MIGHT change. When you play with a custom hotrod like a 257 STW these are things you have to consider. I've played with several 6.5 hotrods and there is always lessons to be learned. Some are harder then others. Sometimes harder on your brass sometimes harder on your pocketbook!
But if this difference is still within normal limits and you are still getting loose primer pockets in ALL brands of brass tried, then pure and simple you are shooting to much pressure for this chamber/brass setup. Don't read me wrong. I'm not trying to be a smart@#%. But if all brass primer pockets open up and everything is within spec, then the brass can not handle the pressure you are putting it through.
So you need to determine do you want the speed/accuracy results that you are getting now and throw the brass away after 2-3 loadings, or do you want to try slower powders or somehow reduce the pressure to gain case life at the expense of possibly not getting the velocity/accuracy you had?
That is the million dollar question right now.
Steve