You don't want a lot of air space with BP and you want NONE with Pyrodex. There really is no reason to go way light on these. Either the MH is safe to shoot or it isn't and frankly accuracy generally will go south in a hurry below 70 grains of BP. Use a non-melting fiber fill such as cotton, carded wool or kapok to take up excess space in the cartridge. Granulated fillers in a bottleneck cartridge like the 577/450 is a big no no.