I purchased a new Ruger 77 in 300 WSM and my hand loads were doing the same thing. I found out that the RCBS dies I were using had to be higher than all of my other calibers. The seating die was compressing the case. Measure just below the shoulder on a resized die with out a bullet and then measure the same place on one with a bullet. If the one with the bullet is wider you are compressing the case with the seating die. Raise the die up. Another problem could be not fully sizing the die. The sizing die should contact the shell holder and just barley cam over.