The parent case for the 7x57 is still 57mm long, and the 308 case is 51mm long. There is no getting around that.
We have a similar comparison here as with the 6mm Remington and the .243 Winchester. Same parent cases. No matter which way you slice it, that 57mm case holds a bit more. For that reason, the 6-(mm Remington)or 7- x57 can always be loaded to a higher velocity with the same bullet than the 6x51 (.243) or the 7x51 (7mm-08).
