Cowtag, you are fine! You only need to worry about volume, not actual weight.
When you hear of someone using 100 gr of black powder or blackpowder substitute (777, pyrodex, BH 209) they are NOT talking about actual weight, they are talking VOLUME measurments.
So,just set your blackpowder measure to 100 gr (if that is what you use) and measure out the powder and shoot. Doesn't matter which of the powders you are using.
THAT SAID: some people want to be more consistent and will measure out their loads, BUT IT WON'T BE 100 GR OF WEIGHT.
For instance: 100 gr (volume equivalent) of BH 209 weighs about 71 gr on a scale. So if you really want to measure loads, you can measure 71 gr to be what everyone is talking about as a 100 gr load.
If you were target shooting, that might make a difference, but for hunting, I have not seen it make that much difference. BH is such a consitently sized powder, you get a very consistent amount each time without weighing it.
So, as sagebrush alludes, to: if you were wanting to use a 120 gr load of BH 209 and you weighed that much out on a scale and shot it....... you could blow something up.
I own 4 blackpowder rifles and have been shooting/hunting for over 20 years and I have never weighed a charge.
txhunter58
venor, ergo sum (I hunt, therefore I am)