Traditionally black powder is measured by volume.
Measuring by weight is more precise, but bringing a powder scale to the range os a pain, and bringing one hunting seems ridiculous.
While a gram of water will weigh a gram on a scale,,,, black powder is not the same. So far my experience has been that my 3f Goex weighs slightly less than my volume measure reads when the charge is weighed on a scale. A charge from my 65 grain volume measure generally weighs a little under 65 grains when weighed on my powder scale. So, not a huge safety concern, but I would try not to mix up loads listed as by weight with loads listed as by volume.