The .275 Rigby is a rimless round also referred to as the .275 High Velocity and was introduced by Rigby for their bolt action magazine rifles in 1907. It is to all intents and purposes the same as the 7x57 Mauser. The original bullet was a 140 grain pointed type which after World War I was changed to a semi-pointed of the same weight. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.