With most muzzle loaders,when you're in the range from 80 grains to 120 grains of powder with a 300 grain bullet that's a good fit(snug/tight) to the barrel, you're burning all of the powder, and getting increased velocities as the charge goes up.

In my Lyman Mustang, with a 300 grain Hornady XTP bullet in a Harvester Crush Rib sabot,I get and average of around 1650fpswith 90 grains T7 FFG, and 1770fps with100 grains. At 100 yards, the 100 grain group shoots almost exactly one inch higher than the 90 grain group. If I up the charge to 110 grains, individual shot velocities start to geterratic, with an average ofaround 1800fps, and my groups start to open up.