To me it looks like a case neck 'donut.'

The brass in the lower part of the neck and shoulder is thicker than in the upper part of the neck. When firing, especially with hot loads, the brass flows from the thicker shoulder to the neck and the brass becomes thicker at the jucture of the neck and shoulder. The problem becomes worse when you FL size the brass by RCBS instructions that tell you to 'cam over' the sizing die which sizes the brass back to SAAMI specs rather than something closer to the size of your rifle's chamber. When the brass is fired again, there is even more case stretching and flow into the neck until the neck is actually constricted and you end up with overpressured loads on your next firing.

Back off on your sizing die- size your brass just enough that it chambers easily, no more. I would start about 1/2 turn or more off the shell holder and size the brass down in 1/16th turn increments until it chambers easily and the sizing die completely resizes the neck portion of the brass. You might have to bump the shoulder back a bit before you get to both considerations, especially if you have a tight chamber on your rifle.

I would just dump the brass with the donut necks in the recycling bin and start over with new brass. You can neck turn them to remove the neck donuts, but the time and expense really isn't worth it for a common chambering with readily available brass like the .243.