Very early in the "proof system" measurement was done by load.
To standardize across various lots, they developed tools so that they could precisely measure the load.
Proof by pressure was a latter development.
That eventually became the piezo method we know today.
I think you already know the answer to this one. If I load a shell with 1 ounce of shot and 40 grains of IMR4350 do I get the same pressure as if I loaded it with 40 grains of Blue Dot? No, because pressure is dependent on the burn rate as well as load. Which was why they had to develop the tools pictured above to standardize the loads between lots.
Pete
