castnblast, said "I don't know of any magical rule of physics that allows a rifle to produce more velocity with the same amount of pressure, all else being equal. "

You may want to qualify that statement. to discriminate between peak pressure and the energy under the pressure curve.
I agree that velocity is a good indicator of pressure, but to really know, it helps to measure and plot the pressure curve over the entire shot.
In my experience you will see many instances where the same velocity will be generated by different maximum chamber pressures.

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