I've had the same thing happen when loading straight walled cases. Is the bullet bulging at the case mouth or is it at the base of the bullet after it is seated. If it is at the base of the bullet apparently your seating stem is occasionly not catching the bullet in alignment as you seat. Since you are belling the cases mouths and placing the bullet as square as you can you are starting out right. Just a suggestion but what I do as I start the bullet, I back off the handle and rotate the shell in the shell holder a little and seat a little deeper and then repeat the process until I have seated the bullet. It might help, it has me. I've been reloading for over 30 years and still learning.
