Muzzle loadeds are charged with powder using a volumetric measure and not by weight. This is done with a flask with a known volume spout or by using a calibrated tool.
The reason being BP does not all weigh the same and substitutes can be quite a bit different, Pyrodex weighs 20% less than BP by volume.

Today it is quite common to precharge vials/phials at home to take to the range. We can get more accurate measures and so more consistant shooting. The volumetric measures are just not accurate enough for the serious target shooter who demands uniformity to within a single particle.

Heres a handy set of charts that will allow conversion between vol and weight

http://www.curtrich.com/BPConversionSheet.htm

Print it out coz it might not be there for long.