weight vs volume
1# of black powder is computed to be 7000 grains regardless of granulation size.
Big guns such as full size cannons etc, loads are normally measured in pounds of BP.
But in small arms (rifles, muskets, revolvers,pistols) (NON cartridge) is measured by grains in volume.
Your owners manual should state the maximum safe grains by volume for your gun.
Because of liability issue, many companies are under rating their guns today.
50 gr by volume does not necessarily translate to 50 gr by weight.
Use a good black powder grain by volume measure and measure each load for consistency.
Unlike smokeless a 1 or 2 grain variance is not likely to be dangerous as in excessive pressure and won't make much difference in your velocity or accuracy.
BP guns are NOT magnums. They don't need to be loaded as such.
Max or near max loads are not necessarily the most accurate.
Start low 70 gr or so and work your way up.
Start at 50 yd get on the paper. do NOT alter powder or bullet.
Fire 2 rounds dial your windage in if needed. Fire 2 more. if windage is still off more than 2 inches, fine tune it.
Then repeat adjusting elevation.
Only change one thing at a time.
When you have this worked in, then increase your powder 5 gr. then repeat your windage and elevation if necessary. then increase your powder 5. keep repeating until your accuracy really fails again. You are now at your max accuracy load. you will likely now drop back to last 5 gr load.
Guess what? you get to repeat some of this at 100 150 yd. But should be quicker and easier.
Now that you have all the above dialed in.
Then start over at 50 yd again with your other bullets.
Long process yes, but it the only real way.