And of course all substitutes are not created equal. The advice from Remington is up to 150 grains of Pyrodex. If you were to try that with B209(after any necessary conversion) you would have much higher pressure, similar with 777.
Then if you used American Pioneer/Shockey's Gold you would have pressures in line with or below Pyrodex. That's one of the reasons the B209 site shows starting with lower charges if one had been shooting Pyrodex, or some if the other substitutes.