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"I have since taken the die apart and polished the inside with 400 wet/dry paper wrapped around a pencil."
400 grit sandpaper sounds too aggressive to me. I would start out with a bore brush and bore cleaner, especially a cleaner that is designed to remove copper fouling. Depending on the die, you might have to use a shotgun or large pistol brush. If that didn't do the job, I would use polishing compound along with a cleaning swab powered by a drill.
Tim
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