For those who think that "BP is measured by volume and NOT weight: a grain is 1/7000 of a pound... and a pound is a unit of weight, not volume (aka: a grain is a grain is a grain, doesn't matter if it is 1 grain worth of flour or powder- Think: a pound of feathers vs a pound of bricks). You can volumetrically meter out any type of propellant or explosive, and if it is uniform in dimensions (spoilers: BP isn't), it can be pretty accurate. BP of a particular formulation gives off the same amount of gas, per it's weight, every time. I've had "100 grain" volumetric measurements be off by as much as 10 grains, it just depends on how the granules fall in (upon testing that, I stopped using volume to meter powder other than to get it close on a scale before trickling the rest in, accuracy improved quite a bit).
As far as deer hunting/plinking/field emergencies are concerned, volume is close enough to work, and it's what most people were using historically (because it's convenient). Just do it the same way to minimize the margin of error.
Another issue with using commercially bought powder measures is that they are marked to meter out modern commercial powder, which has been generally standardized to a density of roughly 1.72-1.80 grams per cubic centimeter. If you have anything outside of that, those measures are not going to meter out close to the marked weight (one of the biggest reasons why "homemade powder (often hand granulated) is less powerful" than bought).
I still often carry a small flask (one of the small pocket flasks) and volumetric measure in my bag, in case my cartridges get wet (I wouldn't know how, but Murphy's law, right?), because I know it'll be kinda-close-enough with the volume (so long as I mark out on the measure where my charge's weight is).