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on my setup (Lee Turret), the 2nd die does the case mouth expansion and powder charge. my dies came with setup instructions, and I started there. I found that I was getting cases where, when seating the bullet, the bullet edge would hang on the case mouth and cause the case to have a ripple or buckle in it. it was happening fairly infrequently. ...maybe 3 out of 100. I decided to adjust the case expansion down (more expansion). this did the trick. all but eliminated the case ripple on bullet seat.

so in my experience, that's what happens with too little expansion. since I started there, then bumped up to the sweet-spot, I can't answer what happens if you have too much.

best of luck with the reloading! I just got dies and a new turret to start reloading .40's now.