Beyond currently published data....I can see that. "Natural limits"?? That is a new expression for me in this regard. I have not seen any load data for the little .25 that specs more than two grains of anything (admittedly faster propellants). These loads are more suitable for the 9mm Luger.
Source of this data? Velocity and pressure data? Type of firearm used to gather the data?
Clark stress-tests and blows firearms up for a living. Once you understand that, it sheds some light on his data.
He really should have a disclaimer in his signature, but we can't force the man to do it.