I have a .50 caliber magnum muzzle loader with a 28" barrel. It has a 1:20" twist. What would be the optimum load of Pyrodex FFP for say, a 300 grain bullet? Does it make a difference if it is a Powerbelt or saboted? I have heard that anything over 100 grains is a waste and ends up as muzzle flash.

You might want to go to this website and get all the info you need. its www.prbullet.com/chart.htm

A 1in20 will need at least 400 grain bullets to be accurate 80 to 100 grains will be your best loads for it I would use Hog/3/F/Trip/7 Loose powder, Don't pound the bullet into the powder !