I hope I'm not overstating the obvious, but just so someone doesn't mis-understand, I'll say it: :
Black Powder DOES weigh the same as it's volume in powder measures; it is the standard, and what powder measures were designed to measure.
Black Powder SUBSTITUTES do NOT weigh the same- they are heavier for the same volume.
As CAYUGAD said, a 100 grain VOLUME of 777 onlyWEIGHED 68 grains. It is not wise to WEIGH out 100 grains of 777 or other substitute and think it's a 100 grain charge. It is actually MUCH more!
On your can/bottle of BP Substitute, it will clearly say that it should be used BY VOLUME as a BP substitute. That infers (but doesn't say) that it should not be substituted by weight.
OK - I'm done - just didn't want anyone to get a big SURPRISE!

IM jaybe