I sorta doubt using large amounts of BH209 with that bullet was the plan. A nice well sealed breach is a blessing when using more corrosive powders too. Westerns load data for 50/90 maxes out at 71gr by weight for a 475gr at over 27kpsi. Thats a compressed load btw.
Real black and most other subs would probably be much less peak pressure. Triple7 probably gets up there too.