Sorry--stepped away for a while because I took the rifle apart and redid the headspacing based on an unfired 6.5 PRC case made by Norma. There's absolutely no doubt that the headspacing was too long IMO; the flow and measurements confirmed that in my mind.
So I decided to take the unfired 6.5 PRC case and put just two layers of magic tape on as a "no-go" and put a bushing in the die that is .001 narrower in diameter for a total of .002 tighter and still not shave the bullet on seating.
Here are the two test cases I just fired with the same loads as the dirty ones above--overall case length growth was .004 and no apparent flow and mouth/neck seal looks much better (and neck diameter is about .004 fired diameter less than the previous stuff above. Now this could vey well be simply the case on one firing is able to "take the punishment" without showing signs of excessive space between it and the chamber--so I'm going to just repeatedly fire these two cases for at least another 3 firings to be sure the spacing problem truly is licked.
I still find it odd that a 6.5 case chambers no problem in a .257 chamber.
