In my opinion, capacity equals velocity.
Completely agree. There's no such thing as a free lunch, nor a magical case shape to help. You might get a more efficient powder burn from one than the other and therefore better long range accuracy, but not likely more velocity. I've been playing with 338/300 WSM's for a while. The case capacity is generally about 5g, �1g, difference in water weight less than 338 Win Mag. While some have equalled the Win Mag in velocity, it had to do with long throats that relieved pressure in the WSM, which have very short throats. I suppose if the Win Mag was also long throated, it would again exceed the WSM.
Some years back I had a custom 338 Win Mag. In order to be able to carry two rifles for a pack-in hunt, and save weight in the ammo, I had a factory rifle rechambered with the same reamer as the custom. Figured that would solve most arguments with an outfitter about weight by having only one set of ammunition. To my surprise at the time, the factory rifle required one more grain of powder to equal the velocity and accuracy of the custom. When I slugged and measured both barrels I had the answer. Don't recall exact numbers now, but the custom was .0004" tighter than the other. It could have been .0007" difference, and there was the difference in velocity.
