hi,
i am new to the board but the problems you are having has happened to a friend of mine.he believes that the moly coat on his bullets may have caused the problem at the neck. last year i had a new ultra light arms 300 wsm built and was told that the chamber reamers were changed three times due to winchester changing case dimensions slightly.
the early reamers caused extreme pressures with the later manufactured ammo. might it be possible that some early die sets are manufactured to the early case dimensions and not fitting your chamber?