It's been my understanding you measure BP and BP substitutes by volume, not weight.
Pyrodex is lighter then BP. For example, in my 45-70, 70 grs of FFG BP works quite well. 70 grs of Pyrodex wont fit.
So I take my measure I use to get 70 gr of FFG and use it with Pyordex, I end up with about 54 grns of Pyrodex.
So I load some rounds of each, some 70 grn FFG and some 54 grs of Pyrodex and run them though the chronograph, I get the same velocity.
I also get the same impact at the target. Same thing with Triple Seven.
I just got started in Muzzling loading but I do the same thing, I load by volume instead of weight. I'm trying to find out what works best in this rifle.
On my in line TC 50 cal, I use 100 grn (by volume) off FFG, I use the same measure for pyrodex and triple seven and I can keep them in the same group (its been too cold to try the chronograph).
All three also seem to shoot the same as two 50 grn pellets.
I know this is all contrary to smokeless powder but its a different game.
Many years ago I met an old guy who was really into muzzle loading/BP shooting. He made his own rifles including the barrels, and they shot, and shot good.
He would use nothing but Swiss Black Powder, he said everything else was junk.
In choosing a charge, he to not to worry about getting too much powder the gun as long as the bullet was setting on the powder. The way he taught to pick a load was to put 'too much" powder in the rifle, lay some newspaper on the ground, get prone and shoot. Not all the powder would burn but would be found laying on the paper. You keep reducing the powder charge until you stop getting unburned powder on the paper. Then you add or subtract tweaking the charge until you get the accuracy you wanted.
At that time I was living in Alaska but before I went to work for APD, so I didn't work in the winter. I lived a lot on small game but didn't have many 22 shells. I had a supply of BP and Pyrodex and a 36 cal revolver, mold and lead.
I did a lot of rabbit shooting with this revolver, it was winter (when I didn't work) and I didn't want to fool with measuring powder with mittens on. I'd just fill the cylinder with powder and push the ball in enough to turn the cylinder. I sill load this revolver that way (some 40 odd years later). The only difference is I have another revolver just like it and shoot it the same way. I also put grease over each hole, I didn't back then because the grease froze before I could load the revolver. I lucked out on that one.
I suppose I could shoot it better if I was picky on my loading, but I can't shoot that revolver good enough to tell the difference.
Now as too my InLine TC. I try to keep it consistent, (using by volume, 100 grns of BP measure for my substitute powder, and so far its pretty dern accurate.
As to a question I asked on this form a while back, I cannot tell the difference in shooting my Inline, the difference between BP 209 primers and regulars 209 shotgun primers.
I'm new to Muzzle loading, so don't take this as gospel, I'm just telling you what I've found so far in my guns.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071