I believe that you are going in the correct direction with making a new expander/belling ram. I ran into a similar situation when I started handloading for the 30 Luger some years ago. My original die set (Lee 3 die set) would not function for me at all. The sizer die would not size the neck down sufficiently to give me any neck tension on .308in. bullets.
At first I thought it was that the expander/powder funnel was too large below the belling area, but that was not it. Anyway, I purchased an RCBS 30 Luger sizing die, then reduced the Dillon expander/ powder through die by .002in. and got sufficient neck tension on these .308in. bullets. I had to machine .040in. off of the base of the Dillon expander/powder die so that it would sit lower and give me enough belling for the case mouth. It works great also on the .309 and .311in. lead bullets. It surely had me scratching my head for a while.........bingo was looking good!!
Out of the 3 dies in the Lee set, I am using only the seating die. We live and hopefully learn.
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