Charlie Sisk performed an interesting experiment a while back. It was in print, not online, so I don't have a link. It had to do with case shape, case capacity and velocity. The twist was he was able to compare two cartridges in the very same barrel.
The cartridges were the 300 H&H and 300 WSM, which despite their radically different shapes have the same capacity. The barrel was first chambered to the H&H. After velocity/pressure testing it was chambered to the WSM and tested. Done in this order each chamber got to use the same length of the same barrel from the case mouth to the muzzle crown.
The results were that the same quantity of the same powder in same size chambers produced the same velocity.
That is gratifying to know. I missed that .300 H&H to .300 WSM comparison by Charlie Sisk somehow.
Thanks.
My own verification of the principle has come from comparing the .458 B&M with 2.240" case length (shortened and necked up RUM case)
to the .458 WinMag with 2.500" case length.
Both made with R-P brass, and gross water capacity is identical.
No magic came from the shorter powder column with same weight of powder.
Throating might be an issue in the Charlie Sisk experiment, if the .300 H&H throat was changed with the .300 WSM throat.
The SAAMI .300 H&H throat is a "leade-only" throat with a 2-degree angle starting at 0.3080" diameter and tapering into 0.300" bore diameter.
Basically NO THROAT.
The .300 WSM has a free-bore diameter of 0.3099" that runs a length of 0.1884" before running into a leade of 1*30' (1.5 degrees).
Maybe Charlie Sisk did them both with the .300 H&H NO THROAT using a special .300 WSM reamer ?
My .458 B&M had a short throat (proprietary), but it was re-throated with a .458 WinMag (SAAMI) reamer.
I used a fat, short cart and a longer cart with same throat and gross case capacities.
I agree with Charlie Sisk's findings.
Everybody here is neglecting throat effects.
They are not insignificant.
Free bore reduces pressure and may be seen as doing so by increasing the effective case capacity
that is containing the burning powder.
Universal gas law: PV=nRT.
Accuracy issues and COL allowed by a longer throat: A different can of worms.