+1 to Carlsen and Nitro John,

The moniker ".275 Rigby" is set in cement now, at least in the USA. Hornady actually head stamped a run of ammo as such, and Ruger has marked the No.1 as ".275 Rigby".
Not historically proper, but also not going to change. Those of us with interest in history and proper language usage can sniff at that

, but it isn't going to alter the gun writers or average blokes terminology.

…...Then there is the bureaucratic mess getting your rifle/ammunition into some of the places in Africa when the ammunition one is using is not exactly as marked on the rifle. I wonder how that works out?

- Mike

The real thing, marked on the receiver ring: "Sighted For Rigby Special High Velocity .275 Bore Cartridge Pointed Bullet 140 Grs"