FYI -The BH209 site claims the powder is non-hydroscopic. Also, on their load data table, they list the charges by volume AND their equivalent weight in grains. I do not think that data would be put there by the manufacturer if the product wasn't designed to be weighed. Too much at stake, IMO. I forget who said it, but it indeed seems like BH209 is smokeless powder with the smoke added.

Anyway, to the OP, if you have a scale, that seems like the way to go as far as eliminaing ONE variable that could cause your groups to grow or shrink.