I experimented with using polyester fill and cast lead rifle bullets some years ago. Had some incidents of rust in the bore when I neglected to clean promptly. Discussing using polyester as a filler with a chemist, he advised that one of products of combustion of polyester was water. I stopped doing it and found accuracy without fillers. Most of my rifles (except a 38-55), shot best with Rx7 powder and no fillers.
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