I first read about this in a series of articles on firearms propellants published by the American Rifleman, back when it was still a Shooter's Mag. As I recall one of the major factors was the Charcoal used. Willow was one of the preferred woods for Charcoal for powder. There was another which was higher regarded but was native to the UK I believe & I forget its name now.
In those days powder makers & made, or had made, charcoal to their specs. I highly suspect that many "Modern" makers of black powder simply use commercially produced hardwood charcoal. Yes it will go bang & put out smoke, but it doesn't reach the peak of that made a century ago.
Look at the velocity specs put out for the 1873 models of .45 Colt & the .45-70 Springfield by the U S Army. Even admitting that today we have solid head cases rather than the old balloon head cases which were used originally & held a tad more powder, the original velocities simply cannot be reached with present day Black.
The old standard 3-1 1/8 12 gauge load gave about 1200 fps @ 15 feet. This is generally un-obtainable with modern powder unless a heavier charge is used. Never-the-less I have always loaded by the equal volume method for shotguns & had satisfactory performance. Volume for 1 1/8 oz of shot will give about 2 13/16 drams of black or 77 grains rather than the 3 dram load of 82 grains. "IF" one points the gun right it will still kill reliably but is not equal in power to the old original loads.
