BP and BP substitutes go by volume not weight. Aside that, when I used Triple Seven, I used Triple Seven primers. There's a body of data that advises against using standard shotgun primers.
BP and BP substitutes are measured by volume and not weight is a misconception.
The weight to or versas volume is tricky when it comes to muzzle loading firearms.
All muzzleloaders need an incremental powder measurer to properly start measuring the amount of powder charge.
Once an accurate powder charge has been developed then if desired a specific fixed measurer can be used.
Example:
A BP shooter has a 60 to 120 grain powder measure.
During range practice the shooter finds that 70 grains measured by volume is the most accurate powder charge.
Notes:
1. Powder Measures Are Not precise, only general, measuring devices!
2. You won't always get the same volume charge each time.
3. Regardless of projectile you use they do not weigh the same, ask any reloader.
4. The only difference between muzzle loading and cartridge firearms reloading is no brass cartridge. Cartridge firearms ammo is not measured by volume but by powder weight.
For hunting and general target practice the science pretty much stops when the most accurate powder charge and projectile is found by the shooter. For competition shooting the science continues.
To insure you will have a consistent accurate charge requires an accurate powder scale, paper, and pen/pencil. Pour 3-5 (or more if you like) charges one at a time weighing each charge separately and writing each weight down. ADD the weights divide by number of charges poured and you have the average charge weight. Using the above example a measured volume of 70 grains is the most accurate load. The average weight should be around 70 grains. If the average weight is considerably higher or lower pour a weighted charge of powder into the powder measure then carefully adjust the measurer up or down till the powder is flush to the top of the measurer then mark the numerated adjustment bar.
This will provide a closer to the same grain weight each time, as if you were to weigh out 70 grains of powder using the powder scale. Or when pouring each charge into a separate vile or into speed loaders.
Note also that not all powder measurers are the same there are a variety of powder measurers made from a variety of materials to include measuring tips for use on flasks and cans, variables in how the shooter pours the charge into the measurer and down the barrel, weather, type powder, powder manufacturer, inadvertently taping the side of the measurer while pouring powder into it, etc, that can affect an accurate charge measure.
Irrelevant to the OP's wanted information, but necessary to mention measured volume vs measured weight relevance.
Using a BP shotgun is a completely different type of critter. Because now the weight by volume of shot has to be factored in, as well as number by size of buckshot. Use of: shot cups, type and thickness number of over powder card(s), thickness of felt or plastic wads, type and thickness of over shot card(s).
The powder charge has to be sufficient to push the load out of the barrel with enough velocity to effectively bust clay targets, penetrate paper targets, or kill birds or other animals.
Example:
12 gauge BP shotgun
Most commonly used starting load 1:1
Set the powder measure to 60 grains of powder, 1each 1/2 inch over powder card, 60 grains volume shot, 1 each 1/32 inch over shot card.
Here the load is around 60 weighted grains of powder with a 60 volume weight of shot.
From here the shooter can experiment by increasing the powder charge, using a variety thickness felt wads, size shot cups, over powder cards, etc. until they get the desired optimum shot pattern.