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The only way to load black powder (or Pyrodex) is to fill the case to the base of the seated bullet - perhaps compressing the powder slightly.
If that is too much of a load, and for a top break I'd err on the side of caution, reduce the amount of powder by 20% to 40% and then fill the rest of the airspace with corn starch or cream of wheat or something else inert. I would avoid anything like granulated plastic, I'm not sure how it would burn and effect the pressure curve.
In no circumstance should you leave any open space in the cartridge or have loose black powder in the case. One gets either squib loads with bullets in the barrel or pressure excursions where the charge detonates catastophically instead of burning rapidly.
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