I have just been going through this with a Dillon 550b. I ended up reducing the amount of bell and lightening the crimp and I have reduced my problems by at least half, maybe 75%. I took a factory round and measured the crimp and then tried to get as close as possible to that figure. It really seemed odd to me that too much crimp would cause this problem and I still don't understand it, but changing it has certainly improved my bullets.