Ahhhh, 'nuther South African black powder shooter. I correspond with a fellow shooter via e-mail almost daily.
He is from Jo'burg.
Anyway here is the scoop on b.p. volume vs. weight.
Black powder is very inefficient stuff. Approximately 1/2 of the burned powder is converted to gases and the other 1/2 stays in solids in the form of fouling of various chemical compounds. The black powder itself is NOT a compound at all but an intimate mixture of the 3 well known base ingredients.
And yes even though all black powders use the same 75, 15, 10 ratio there is a great difference in brands of black. I'm not real sure but I'll venture to say all you can get where you live is Wano, a German powder, though some Swiss might be available now.
The difference in powders varies as the HOW the mixture is formed and the purity of the saltpeter, sulphur and the kind of wood used for the charcoal.
Also the moisture content of the powder can affect performance.
Back to your question.
Since black powder is inefficient compared to modern smokeless it is not load sensitive. You, your firearm, or chronograph will never tell the difference between 80 or 83 grains by weight.
In large calibers it is unlikely you would notice any difference at all in even five grain increments.
That is why volume is the easiest way to measure your powder.
It makes no sense to weigh, for instance, exactly 83 grains each time you load as the performance will not vary from 77 grains to 86 grains, or 90 to 95 grains and so on.
Now there will be a noticable difference in very small bores, such as the little .32's,36's, and 40's with a five grain spread, but not really very much.
Just for giggles try this. Set your volume measure to, say 100 grains, then dump the charge on your scale. It might weigh 110 grains or even 90 grains, but the trick here is to weigh ten volume charges in a row and I'll bet they will weigh plus or minus 3 grains each time.
In calibers over .45 you will never notice the difference in that small amount of variance.
Now in the modern smokeless world 5 grain variance is huge, but not in black powder.
You folks having any luck getting muzzle loaders deregulated?
In the U.S. we need not apply for a license to buy black powder weapons. We should all feel very lucky here.
Hope this helps.
Let us know what type of black powder weapons you shoot/hunt with and so forth.
And good to hear from you.
Cheers,
Maxi