There is no volumetric unit of grains avoirdupois - it is a unit of weight. We most often measure smokeless powder by volume, but we have to calibrate the measures for each powder because they differ significantly in density and drastically in burn rate and sensitivity to differences in charge weight. Black powder varieties differ much less in density from each other, and are much less sensitive to small differences in charge weight. B.P. substitutes are of different densities and, to some extent, burn characteristics but these are known (approximately - "15% less" than what specifically?), but one can calculate for the difference in density. Both for smokeless powders and for B.P and substitutes, volumetric measurement is sufficiently precise for many uses, or one may need to weigh charges for higher precision.
Different makers of B.P measures use different standards for calibration (usually a particular make and granulation, an average of certain varieties, or the density of water, and for some particular measuring technique - as-poured, poured and struck off level, tapped to settle and topped up, or settled-&-topped and struck off) but well-made measures (not all are) will all throw approximately the weight they are marked with of any variety of B.P. and measuring technique. How much a particular measurement differs from the indicated weight depends on both the differences in density of the particular powder (make, granulation, and lot) and in measurement technique from the makers calibration standard. For some shooters, a poured measure of any FFg is close enough, while for others, the difference in density and burn rate between different lots of a particular granulation from a specific maker may require different charge weights.
I'll go back to the original poster's question about what is the nominal capacity of his measure. Filling the flask with some variety of B.P. and filling he spout using his usual measuring technique, then weighing that charge will give a more reliable answer than comparing it to some other volumetric measure of unknown accuracy (may are good enough, some are not). Doing it with multiple varieties of B.P and averaging the results will give a more general answer.
Regards,
Joel