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The 173-grain round nosed bullets need a longer throat than the concise 140- and 150-grain spitzer bullets require, so the H.V. throat was shortened to deliver the best accuracy with the lighter bullets. If you have a H.V. chamber, you will find that the 173-grain bullets won’t chamber in your rifle (or at least shouldn’t, if the chamber was cut properly).
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Not all .275 Rigby chambers are of the High Velocity variant, but if you are shopping for a .275, be sure and verify which chambering it is. I
The throat differences, true or not?
All pre-WW2 .275 Rigby rifles were first proofed at the Oberndorf proofhouse as 7x57. So their chambers conformed to he now CIP standard for that cartridge. But when Paul Roberts registered “his” .275 H.V. Rigby with the CIP in 1984, he reinvented the cartridge and supplied a different data table. So now CIP has two different data sets for both otherwise interchangeable cartridges. While a minimum 7x57 barrel still has to have a throat length of 19.20 mm, a .275 H.V. Rigby chamber may have a smuch shorter throat of just 5.65 mm. On the other hand, the 7x57 maximum pressure is still 3900 bar, while a .275 H.V. Rigby is assigned a 18% lower maximum pressure, just 3200 bar. So, if you shoot 7x57 loads in a .275 H.V. Rigby rifle, the rifle is legally out of proof. Mr.Roberts and Mr. Little made the same blunder when they redesigned the established 12.7x70 Schueler, the old .500 Jeffery, with a different shoulder and created their “New .500 Jeffery”.
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Re: Behind the Bullet: .275 Rigby..