218bee,
You don't need a forming die. Just run the '06 cases into your 7.7 die and trim to length. The reformed cases will be around 1/4" long. I carefully grind them most of the way down on the side of a fine grit bench grinder wheel and toss them into water (use gloves as the cases get hot). Then finish trim them on my Lyman Power Trimmer.
HTH