Something in the back of my mind makes me seem to remember that the 35 Brown Whelen uses the 280 Rem case as the base case. The 280 Rem's shoulder is further up the neck ( shorter neck). It has a different headspace than the 30-06 /270 Win ( which I believe have the same headspacing).
You may want to try some 280 Rem brass in forming your next batch of 35 Brown Whelen cases. Maybe that will help.
