All substances - note use of the term - used as propellants with a small p - like Black Powder, are designed to fill a container of a certain capacity. As such, they are measured by volume. However, the volume of 'stuff' also has a weight, which is measured in grains avd. So a case of a certain size, when it also holds the bullet, actually contains a measure of powder that measures, fer'instance, 70gr.
Soooooooo, what I have been doing for many years, is to initially weigh a BP charge on a good set of scales of one kind or another - your choice - pour into a powder measure, and then adjust it so that the charge is level with the top. That results in the volume of powder that you require, and also provides a repeatable weight by volume, of that particular grade of BP. I use the word stuff since BP is NOT a propellant, but a Class 1 explosive, unlike the numerous BP substitutes, that are classed as propellants.
In addition to requiring more application of contact heat to ignite them, hotter caps is the usual way, these substitutes also physically weigh LESS than an equal volume of BP, which is why you must reduce the amount that you use by the figures given in the manufacturers' data book.
Approximately 10% for any of the grades of Pyrodex, and 15% for Triple 7 and Cleanshot. So a 30gr load of BP in your revolver will occupy the same space when you use Pyrodex P, but it will only weigh 26-27gr. That same load in Triple 7 will only weigh 25gr, or thereabouts.
The substitutes are also more efficient in the production of propellant gases, or 'vigorous' in physical terms, so that they will produce a higher velocity than an equal weight of BP. I've chronographed my BP loads of round ball in my Ruger Old Army, and the load I settled on about 25 years ago that works well for me is 26gr of Pyrodex P - that gives me around 885fps. The same VOLUME load of FFFg BP gives me around 795fps.
Hope that helps.
tac