All of my findings are from practical use over several decades of shotgun shooting ,W.W.Greeners writings are mostly of the nitro period of early breech loaders .
Feltwad
BP was still widely used when my 1896 Sixth Edition of "The Gun & Its Development" was published.
pgs 528-529:
"Of late years it has been the rule to use large grain-powder, as No.4 or 6; only a few of the most conservative sportsmen retaining the old-fashioned fine-grained powders.
For general use in 12-bore guns the No.6 is too large and not sufficiently quick in its action."
The paragraph that follows this explains that coarse grained powders, like #6, pattern better but can produce lower velocity. "The small grained powders, whilst giving great velocity, generally cause the pellets to scatter much more rapidly..."
C&H #4 from what I can learn, is about like FF Goex and maybe a little coarser than FF Swiss. No.6 is about like FG Goex.
I take this to mean that they do not recommend fine grained powders.
Dan