Prvi partisan load 9.3x62, it's cheap.

But as you said, you're not after metric.. If its only an 8x57, how about a .358 Winchester? Very efficient, and quietly chugging along despite not much attention.

Rigby just whacked it's name on it to sell rifles by them, in a cartridge named by them. At least that's how I view. I like to say 7x57 to people when they talk about the .275 rigby to annoy them