I have a .275 BA Rigby, built in 1901. I believe that when Rigby introduced the 140grn (or was it 145grn? bullet - I could look it up, but I'm lazy) - they called it ".275 Magnum".
I use commercial 7x57 ammo, or military surplus, or handloads. They all chamber & extract perfectly, with no evidence of fireforming.

Does anyone know if Rigby changed the rate of twist when they introduced the lighter projectile?