I have a 1:48 twist Firestorm TC flintlock, and I had to crank the powder charge UP to get it to shoot Powerbelts well. As a matter of fact, anything but roundballs shot better with a max. charge of 150 gr. of black powder and the synthetics, rather than less. And I bolted a scope on for my accuracy testing. Roundballs it likes at about 80 gr. BP. Don't try this with muzzleloaders not rated for up to 150 gr. loads! The recoil from 150 gr. is a bit of a bear from the bench for us mortals, but I don't notice it when standing and shooting at a deer. The accuracy demanded it. And so far this seems to hold true in my gun w/ the Powerbelts from 245 to 348 gr.s. Good luck. Your gun, and mileage may vary.
My experience, and that of others, with the Powerbelts is that you usually don't get full penetration, and if you hit a bone, there is MAJOR damage. Kills 'em right now, w/ all the energy expended in the deer! Haven't been able to get a long shot yet to see if they expand less violently at longer distances and lower velocities or not.