If you are sure the cases were not like that before you seated the bullets, I have seen the sizing die do this to cases where the case was over max length and the top of the neck was being pushed down by the sizing die. The seating die will also cause exactly the same issue if the neck is too long or if you have the seating die screwed down to far and the crimp part is engaging on the longer brass. Since there is no cannelure in the bullet, it forces the neck down and bulges the shoulders. Trim your brass to correct length and screw your seating die OUT 1/2 turn.
|