Hello John,
British Sporting Rifle Cartridges have the following listed for this round:- 577/500 No.2. Express.
1.
Black Powder. 130 grains, (Probably Curtis & Harveys Fg)my brackets.
Bullet weight 340 grains, lead, paper patch, round nose, copper tubed. Overall length 3.384". Case length 2.840"
2.
Black Powder 130 grains,
Bullet weight 380 grains, lead, paper patch, round nose. Same case and o/a cartridge lengths.
3.
Black Powder 130 grains,
bullet weight 300 grains, lead, paper patch, round nose, hollow point. Same case and o/a cartridge lengths.
Nitro loadings are given but using powders that haven't been made for fifty or more years.
With any old rifle, I'd suggest you stick to BP, you can't overload a cartridge with that.
Harry