.577 BPE will normally shoot 500 to 650gr. bullets. With such, a slow twist will work fine- I'd think something from a 36" to 48" should suffice. Even slower will work as my .58 Hawken (66" twist) shot 675gr. RN bullets into 2" at 100 meters with 165gr. 2F.

I shortened the plunger of the Lyman #57730 mould creating a short hollow base with a very thick skirt, increasing the weight by just over 100gr. This mould is probably undersize for a BPE - but of course, much depends on the groove diameter of the available barrels.