Charlie,
A few years after you ran the .300 H&H/WSM test I had some very interesting conversations with guys who run major pressure labs about case-shape affecting pressures. NONE of them had ever seen that in any of their testing--in other words, the same amount of powder room produces the same pressures and velocities, just as you found.
However, one of the guys (the oldest and most experiences) said that case shape did affect the consistency of powder burn, especially shorter cases with shoulder angles around 30 degrees, which is of course the modern ideal for "accuracy" rounds. Both shallower and steeper shoulders (including the 40-degree Ackley angle) resulted in less consistent pressures.
The notion that the .300 WSM's case-shape allowed it to produce more velocity due to "efficiency" of the powder burn was promoted by Winchester PR. Their major "evidence" was the fact that the smaller WSM case could achieve 2960 fps with 180-grain bullets, the same as the then-standard factory load for the .300 Winchester Magnum. But they apparently failed to notice (or decided to ignore) the fact that the .300 WSM required more pressure to match the .300 Winchester load. Since then, of course, SAAMI has approved even faster 180-grain loads for the .300 Winchester Magnum.
