More on reloading that I almost forgot.
When you use a powder dispenser, it has a micrometer adjust, and the good reloader checks the charge on a balance while adjusting the micrometer to insure he/she is on the money so to speak. So, in effect, you have a micrometrically adjustable 'teaspoon' which you are expected to calibrate by weight for verification, and which performs optimally by slicing off excess chunks of powder to get an even level amount with each turn of the dispenser.
In addition to this, the manufacturer publishes sets of weight/volume tables in rather largish books.
And finally, the user (reloader) is expected to test the load by firing groups of loads for test purposes.
I'm glad this was brought up, because it highlights the fact that only when you have a uniform material of known density and solid form can you be sure of getting the right amount of powder by volume.
At Kodak, we dispensed solids by weight using strain guages under the container. These guages are highly sensitive to weight and are set to sense the exact weight less the tare value of the container. We do not use volume for solids.
In many situations, we even use weight for liquids.
As noted above, volume changes with the 'fluffing' of a powder, but also varies with temperature in some cases or with humidity. A caked solid has more packing density than a fluffy quantity of the same chemical. It therefore delivers more weight / volume.