SOme of you may have notice that the early 56-50 dies sets from Lymann had a problem.
The expander plug was not tapered correctly for Starline brass, and would deform the case outward if you used it deep enough to bell the case mouth. I believe it was just a 50-70 expander plug turned to .512. I had to shorten and grind on my original set. The good news is that current production set I just got fresh from Tammy, who got them fresh from Lymann has corrected this. If you have the old style, I would give them a call, and they should send you the new expander plug.