A pound of something weighs 7000 grains, no matter what it's made of;
black powder, pyrodex, lead, hamburger, cheese, whatever . . . . .
The conversision factor between the two systems of measure is 7000-to-1.
Now, most people do use a "powder measure" when they charge their muzzleloaders. (Who wants to carry a delicate scale out in the woods?) These "measures" were originally set up for BLACK POWDER. So that, when you filled up a 70 grain volume measure, it gave you 70 grains in weight. A slight variation will occur with different granular sizes; 4F will weigh more, Cannon powder will weigh less. But for 2F & 3F, it'll be pretty accurate.
But, Pyrodex is a completely different chemical, (actually a smokeless powder with "smoke" producing agents added), and therefore doesn't have the same density as black powder. Pyrodex is less dense, or "lighter". Because there were all those old black powder measures out there, the gun-powder manufacturing companies fiddled around with some different filler-chemicals, and bonding agents, etc. until they got Pyrodex to "measure out" in volume, an equal energy level of propellant, as black powder.
So, if you use Pyrodex, (I can't imagine why, HA-HA!), a pound of it will still weigh/contain 7000 grains. But when you use a black powder measure, it'll throw a lighter weight charge, and you'll have some Pyrodex left over, when you run out of caps & balls.
Besides, to paraphase Iggy, "That Pyrodex ain't never gonna catch on, anyway."
Hence the title of this forum; BLACK POWDER and Cowboy Action Shooting.
Watch your top-knot.