What I mean by this, is that it doesn't exist.

There is no such thing as a .275 ""Rigby"" cartridge.

Its a name made up by modern American gun writers from a misunderstanding. The cartridge was just called a .275. The rifle was called a Rigby-Mauser. Rigby sold Mauser rifles under that brand name.

The 7mm Mauser cartridge was never a Rigby proprietary cartridge (it was military for several countries). Rigby did not sell it as the .275 Rigby on their own brand of ammo , they called it the .275 bore, and they stamped their rifles the same. There is no mention of .275 Rigby in any of their catalogs - or by anyone else. The earliest reference is from an American gun writer in 1970.

The two most famous users of the .275 / 7mm Mauser would have to be WDM Bell, who famously shot 800 bull elephant with one and Jim Corbett of India who shot at least one man eating tiger with one. Both of them referred to the cartridge variously as the .275, the .276, the 7mm, the 7mm Mauser or to the cartridge and rifle together: the .275 in a Rigby-Mauser.

British website and auction houses are often more correct and dont fall for this (apart from Rigby themselves, who must be delighted).

It started with Jack O'Conner with an article in 1970, where he mistook the Rigby name as referring to the cartridge and not the rifle, and people have been running with it ever since. It's now all over the internet as if it's a valid thing, and no article on the 7x57 is ever innocent of mentioning it. Even responsible people like Craig Boddington and John Barsness, have all fallen for this. Hornady have brought out 7mm brass headstamped .275 Rigby. Even Rigby themselves (the new company) - who should know better - now sell ammunition headstamped .275 Rigby (which they may not, they all seem very young.) Original .275 Hi-velocity cartridges of the early 20th century are stamped Roth, which was the German made ammunition that Rigby repackaged and sold as their own.

Meanwhile sundry people are building 7x57 custom rifles and having them marked "".275 Rigby"" because they like the traditional British old-world sound of it.

It's essentially become a true thing. And it sounds like it should be right. People believe it when they hear it. The power of the internet has made it so. But its just a .275. Or a 7mm Mauser.