As Mike said, just load it volume for equal volume of shot. Don't worry about what it actually weighs. Pyrodex is a bit harder to light in a ML rifle with a complicated path for the spark to travel but it lights pretty well in a ML shotgun with a nearly straight through flash channel and does just fine in shotshells. Pressures will be fine and safe. Good way to use it up. Mike's charge obtained with the measure set for 75 grains and actually weighing 55 grains is quite normal and right in the ballpark. That's why they recommend using Pyrodex by volume with BP. The Pyrodex will weigh 20% or so less. If you used the same actual weight of Pyrodex, you'd have a stronger charge than with the BP.
