Thanks guys, I get it now. I have some moulds with bullets in the 465 to 500 grain range already too, so this might work out well.
Are there jacketed bullets in this weight available? I have used cast bullets of this weight a fair bit in my Sniders but have never even looked for a jacketed bullet for that purpose.
I also have a .577 NE 3" project gun under construction. It is a heavy barrel single shot on a Riedl falling block action. The action is massive and has never been made up into a rifle so is in new condition though it was made in the early 70s. The barrel I have fitted is long and heavy to give it some weight. It has only been test fired to this point. It lost priority once I got the Jeffery double.
Regarding the 75% rule, the first ammo I loaded for the .577 double was with a bullet of about 485 grains. It was with a lighter powder charge though. It did shoot well and consistent, but it shot a lot higher than the point of aim at 50 Meters. I wonder if I go back to the heavy charge with these lighter bullets, if they will come down to my point of aim. It would be worth a try and I am sure they would be more pleasant to shoot than the heavies.