Most black powder is measured in the field volumetrically. The volume measures are marked, based on an average grain weight per that volume.
There are problems in using a scale to measure black powder at home. Not the least of which is the possibility of a static electric spark setting it off. Black powder is FAR more easily ignited than smokless powder
Mark Lane to William Buckley: "Have you ever referred to Jessee Jackson as an ignoramus?"
Buckley: "If I didn't, I should have"