In my opinion the idea of a tapered case being less likely to cause extraction problems is incorrect, especially in double rifles. My Krieghoff .458 DR extracted flawlessly, but the same rifle, with .375 H&H barrels fitted, had real extraction problems.

My analysis was that the action flexed slightly at the moment of highest pressure, creating the equivalent of slightly excessive headspace, then sprang back, wedging the tapered case firmly in the chamber. I think that the perhaps counterintuitive universal experience is that parallel sided cases extract easier.

The real benefit of gradually tapered cases is in feeding. Both the .375 H&H and its progeny, the .300 H&H, feed flawlessly through Mauser type double row magazines.