Some powders are sensitive to the position of the powder (when not a full case of it) in relation to the primer, hence why the filler is required to keep the powder back by the primer. I use IMR4198 for reduced recoil youth-type loads in rifles cases, and that powder is not sensitive to the position in the case with reduced loads. Its not a pistol powder as such, but is fast burning, and is commonly referenced for use with cast bullet reduced rifle loads.
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