Don't believe it. Those spouts are notoriously inaccurate. Weigh a charge of real black from it to see what it really throws.
I don't "weigh" any powder for my BP guns as it's my understanding charges are measured by volume. I would bet most any BP substitute measured by volume would not weigh the same as real BP...or am I missing something?
Anyway, I can certainly check what the spout measures against my adjustable BP measure.
Yes, BP powder charges are commonly stated in 'grains/volume'. Which leads to a problem. BP volume powder measures are inaccurate; there is no 'grains/volume' standard by which they are calibrated. (There is a 'grains/weight' vs 'grains/volume' standard, that being water under standard laboratory conditions, but that's beyond the scope of this thread.) I have 4 adjustable volume measures, two fixed volume measures and several interchangeable spouts. I haven't compared them all, but the 4 adjustable measures vary by as much as 15% depending on the charge.
So, we see people saying they use XX grains and get great accuracy. How accurate is that XX grains? My experience tells me it could easily be off by 15% - so is it 90 grains or 77, or maybe 103. Your '24' grain/volume spout could be 20 grains/volume or 28 grains/volume.
In the end, however, the actual number isn't important to you (with one exception I'll mention later); what's really important is consistency. Use the '24' grain spout if the amount it throws gives good performance, and forget about whether or not it's accurate.
However, if you're trying to work up an accurate load, based on someone else's experience
, assuming that spout is really throwing 24 grains could lead to confusion. In this case you really need to know what the charge you're trying to match really WEIGHS, and what the actual WEIGHT of powder your measure throws is regardless of what it says.
Finally, you are correct that the bp substitutes are less dense than real black powder and any weights you obtain must be adjusted if you need to know what their black powder equivalents are. There are conversion data published in several internet archives to help achieve that. When dealing with charges by weight it's important to know what the powder is so you can make the conversion if you're using a different powder. If you attempt to get around having to use that conversion by using grains/volume you must then deal with the inaccuracy of the various volume measures.