I'm going to rain on the Volume Measure parade here.
Traditionally, yes, it was and still is measured by volume, BUT... for that to be an accurate method of using a certain amount of powder in a given space, the "grains" of powder must be uniform along with the measure being consistent and how the powder is dispensed being consistent. Problem is, powder grain size isn't terribly uniform especially in the cheaper grades of powder. Powder measures vary. How one guy dispenses will not be how the next guy dispenses. And to add the icing to the confusion cake, that 7000gr to 1lb is a WEIGHT measure NOT volume. Think I'm offbase? Get an accurate scale and your volume measure. Set your volume measure and pour a charge. Weight the charge. Put it back and pour another but this time, tap the volume measure a couple times and pour till it's just as full. I'd bet the farm the two are very different because of settling in the second method.
So if you want accuracy, you will be WEIGHING charges. You can switch to volume after you develop a methodology to be extremely consistent in how you throw your charge. If you're using cheapo powder, this gets critical fast if you want accuracy because in cheapo powder, the amount of "fines" goes up fast. Fines are powder grains that are smaller than the nominal size. Explanation- "fines" in 3f are probably 4f or smaller. Fines in 2f are probably 3f, 4f or smaller. That stuff still burns and contributes to the bang so it must be accounted for in making good ammo, hence weighing the charge. This is also a significant part of the explanation for velocity variation in cheap v expensive powder. When I'm assembling competition ammo, I have a measure set for each gun. For musket, I'm shooting 42g 3f Old E. I weigh the first charge, then use the powder measure to throw the next 9 for a total of 10. I weigh the next charge and throw the next 9 for another total of 10. So I'm weighing every 10th charge to check for consistency. I also have a set routine in how I swing the throw lever on the powder measure/dispenser because how that is operated will affect the weight of the dispensed powder. Accuracy is about variable control.
You can get by just fine for plinking by nothing but volume measurement, but when you get into the accuracy weeds in competition, that volume thing will go away quickly.