Since pressure pushes in all directions, if two cartridges have the same capacity, the one with the wider case mouth will allow more energy to be transmitted to the bullet (which is why a .458 WM gets 5,000 foot pounds and a .257 Wby doesn't).
If you could push a 100 grain bullet out of a .458, you could get far higher velocity than with a .257 Wby, at the same pressure. Problem: The BC would be ridiculously small. Possible solution: a .257 caliber 100 grain bullet fired out of a .458 with a light weight sabot.
The correct twist and powder would have to be selected. Accuracy might be a problem as the sabot dropped away. I don't know.
About 50 years ago Remington marketed a sabot concept, 55 grain .224 bullets in 30-06 and 30-30. I don't know how accurate those were. They failed in the market but then again a lot of Remingron stuff failed due to incompetent marketing.
